Geological Setting and Geochemistry of Oil Shales ... - ACS Publications

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G e o l o g i c a l S e t t i n g a n d G e o c h e m i s t r y of Oil S h a l e s i n the P e r m i a n P h o s p h o r i a F o r m a t i o n EDWIN K. MAUGHAN

Downloaded by FUDAN UNIV on December 9, 2016 | http://pubs.acs.org Publication Date: August 1, 1983 | doi: 10.1021/bk-1983-0230.ch011

U.S. Geological Survey, Denver Federal Center, Denver, CO 80225

The Permian Phosphoria Formation in the northwestern Interior United States contains two phosphatic and organic-carbon-rich shale members, the Meade Peak Phosphatic Shale Member and the Retort Phosphatic Shale Member. These rocks were formed at the periphery of a foreland basin between the Paleozoic continental margin and the North American cratonic shelf. The concentration, distribution, and coincidence of phosphorite, organic carbon, and many trace elements within these shale members probably were coincident with areas of optimum trophism and biologic productivity related to areas of upwelling. In the Phosphoria sea upwelling is indicated to have occurred by sapropel that was deposited adjacent to shoals near the east flank of the depositional basin. Maximum organic-carbon concentration is as much as 30 weight percent in the organically richest beds in the shale members and the maximum average in each member is about 10 weight percent. A close association occurs in the distribution of the organic carbon, silver, chromium, molybdenum, nickel, titanium, vanadium, and zinc. Phosphorous differs slightly from the distribution of organic carbon and by contrast seems typically associated with copper, lanthanum, neodymium, strontium, yttrium, and ytterbium. Subsequent burial of the sapropelic muds by Triassic and younger sediments and the consequent rise in ambient temperature has led to catagenesis of hydrocarbons from the kerogen in these rocks. In some areas of southwestern Montana, however, burial has been minimal, temperatures have remained low, hydrocarbons have not been generated, and these rocks remain oil shales that have the potential for producing synthetic o i l and gas. This chapter not subject to U.S. copyright. Published 1983, American Chemical Society Miknis and McKay; Geochemistry and Chemistry of Oil Shales ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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GEOCHEMISTRY AND CHEMISTRY OF OIL SHALES

Studies of the Meade Peak and the Retort Phosphatic Shale Members of the Hiosphoria Formation i n i t i a t e d i n 1973, have i n v e s t i g a t e d the organic-carbon content and some aspects of hydrocarbon g e n e r a t i o n from these rocks. The Phosphoria Formation has been p r e v i o u s l y s t u d i e d c h i e f l y because of i t s phosphorite resources. Phosphorite has been mined from the Retort and Meade Peak Members i n southeastern Idaho, northern Utah, western Wyoming, and southwestern Montana. P r i n c i p a l s t u d i e s regarding the phosphorite deposits have been produced by M a n s f i e l d (1) and by McKelvey and others ( 2 ) . The o r g a n i c ^ c a r b o n - r i c h ( s a p r o p e l i c ) mudstone beds a s s o c i a t e d w i t h the phosphorite i n these two members a l s o have been the n a t u r a l sources of petroleum t h a t occurs i n t h e r e g i o n . Determination of the content and d i s t r i b u t i o n of organic carbon has been reported by Maughan ( 3 ) , and an e v a l u a t i o n o f t h e hydrocarbon production has been given by Claypool and others (4). I n southwestern Montana, however, b u r i a l was l e s s than 2 km, ambient temperatures remained low, and the kerogen has not formed hydrocarbons. In t h i s area i n Montana, the kerogen i n t h e carbonaceous mudstones has r e t a i n e d the p o t e n t i a l f o r hydrocarbon generation and the Retort Member i s an o i l shale from which hydrocarbons can be s y n t h e t i c a l l y e x t r a c t e d . A b r i e f e v a l u a t i o n of the o i l - s h a l e p o t e n t i a l to produce hydrocarbons was r e p o r t e d by Condi t (5_). Paleogeographic i n t e r p r e t a t i o n s by Sheldon and others (6_) and by Maughan (7j-8) are important to the understanding of the d e p o s i t i o n a l s e t t i n g of these rocks. A n a l y t i c a l trace-element data have been presented by Gulbrandsen (9-10) and by Maughan (11), and a comprehensive b i b l i o g r a p h y to published g e o l o g i c a l and chemical data f o r these rocks (9) provides sources of much o f t h e a n c i l l a r y i n f o r m a t i o n about the Phosphoria. The present report summarizes the r e s u l t s of recent hydrocarbon-source-rock s t u d i e s of the Phosphoria Formation. Most of the data and i n t e r p r e t a t i o n s i n t h i s r e p o r t concerning the r e l a t i o n s between the phosphorite and the petroleum source beds have been p r e v i o u s l y published by Maughan ( 1 2 ) , but are i n c l u d e d here because of the l i m i t e d a v a i l a b i l i t y of that p u b l i c a t i o n i n the united S t a t e s . Paleogeographic S e t t i n g The Phosphoria Formation was deposited i n a f o r e l a n d b a s i n between the C o n t i n e n t a l margin and the North American c r a t o n i c s h e l f . This f o r e l a n d b a s i n , which i s here defined by the area of d e p o s i t i o n of the two organic-^carbon-rich muds tone members of t h e Phosphoria ( f i g . 1 ) , has been named the S u b l e t t b a s i n ( 8 ) ; and i t covers an e x t e n s i v e area of approximately 400,000 km (about 700 km by 600 km). The b a s i n has a northwest-southeast-trending a x i s and seems to have been deepest i n c e n t r a l Idaho where deep-water sedimentary rocks e q u i v a l e n t to the Phosphoria Formation are exceptionally thick. The depth decreased toward the shelves and land areas i n d i c a t e d i n f i g u r e 1. The deepest p a r t of the S u b l e t t 2

Miknis and McKay; Geochemistry and Chemistry of Oil Shales ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


Figure 1. Sublett basin (solid line, dashed where uncertain) in relation to major Permian structural and paleogeographic elements. Areal extent of Meade Peak indicated by left diagonal rules; Retort, by right diagonal rules; limit of middle Permian epicontinental shelf deposits indicated by dotted line. Crosssection A-A' shown in Figure 3, B-B' in Figure 2.



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b a s i n l i e s approximately i n the a x i a l bend of the C o r d i l l e r a n miogeosynclinal trough shown by Roberts and Thomasson (13)» The southern l e g of the miogeosyncline trends approximately N. 30° E. i n Nevada, and the northern l e g trends approximately Ν· 40° W. i n northern Idaho. Shelves that f l a n k e d these legs of the miogeosyncline l a y adjacent to the T r a n s c o n t i n e n t a l arch and t o the Canadian s h i e l d . In c e n t r a l Idaho, Permian rocks b e l i e v e d to be equivalent to the Phosphoria Formation comprise about 2,000 m of probable deep-water, t h i n , planar-bedded, s i l i c e o u s mudstone and limestone; but on the s h e l v e s , equivalent s t r a t a comprise about 100 m of shallow-water deposits of mostly limestone, dolomite, and sandstone. The northwestern l i m i t of the S u b l e t t b a s i n may have been the open ocean, or the b a s i n may have been s e m i - r e s t r i e t e d by an o f f s h o r e v o l c a n i c i s l a n d - a r c complex t h a t separated the sea i n the f o r e l a n d b a s i n from the open ocean. The Lower Permian, probably Wordian, Hunsaker Creek Formation of the Seven D e v i l s Group along the Oregon-Idaho border (14) represents a v o l c a n i c i s l a n d - a r c complex that could have been an oceanward b a r r i e r to the S u b l e t t basin (12). However, the Seven D e v i l s Group has been i d e n t i f i e d as a part of W r a n g e l l i a , an e x o t i c terrane accreted to the North American p l a t e a f t e r P a l e o z o i c time (15). I t i s conceivable that a more northern s e c t o r of the same a r c h i p e l a g o , or another of the v o l c a n i c - a r c terranes i n d i c a t e d by Davis and others (16), had been l o c a t e d adjacent to the S u b l e t t b a s i n but i s now p o s i t e d elsewhere, p o s s i b l y as a part of the Wrangellia accreted terrane now l o c a t e d along the British Columbian to Alaskan c o a s t a l margin. The southwest l i m i t of the Sublett b a s i n i n Nevada was along the Humboldt highland (17). The Humboldt highland was the p r i n c i p a l source of terrigenous sediments incorporated i n the Murdock Mountain Formation (18), which i s equivalent to the lower p a r t of the Phosphoria Formation ( f i g . 2 ) . Shelves on the periphery of the S u b l e t t b a s i n were mostly areas of carbonate bank sedimentation. The Plympton and the Gerster Formations compose the dominantly carbonate rock deposits of the Confusion s h e l f at the south edge of the Sublett b a s i n . The Park C i t y Formation comprises the carbonate bank deposits on the Wyoming s h e l f at the east edge. Some p a r t s of the shelves adjacent to Permian land areas were areas of l i t t o r a l sand deposition that c o n t r a s t w i t h the area of carbonate and t e r r i g e n o u s mud d e p o s i t i o n . A land area i n Montana, the M i l k River u p l i f t , was provenance f o r l i t t o r a l sand deposits of the Shedhorn Sandstone along the northeast edge of the b a s i n . Tectonic subsidence of the Sublett basin i n p a r t of the r e g i o n seems to have provided water deep enough f o r u p w e l l i n g c i r c u l a t i o n , and to have created a change i n d e p o s i t i o n from r e g i o n a l carbonates and l o c a l sandstone i n t o a more complex d e p o s i t i o n a l p a t t e r n that i n c l u d e d the accumulation of the mudstone-chert-phosphorite f a c i e s that c o n s t i t u t e the Phosphoria Formation. High b i o l o g i c a l p r o d u c t i v i t y and the accumulation of



Figure 2. Cross-section B-B' showing generalizedfades relations and nomenclature of middle Permian strata across Sublett basin from northeastern Nevada to northeastern Utah. (Section location shown in Figure I.)



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s a p r o p e l on the sea f l o o r are a s s o c i a t e d w i t h contemporary c o a s t a l upwelling (19), and s i m i l a r environmental and depositional conditions i n Permian time are attributed to the rich accumulations of organic matter i n the Phosphoria Formation. The Phosphoria Formation was deposited on the f o r e s l o p e between the carbonate and l i t t o r a l sand d e p o s i t s of the s h e l f and the dominantly cherty mudstone sediments of the a x i a l p a r t of the b a s i n . Winds blowing t a n g e n t i a l l y o f f s h o r e across the Phosphoria sea from the M i l k River u p l i f t , augmented by the C o r i o l i s f o r c e , c a r r i e d surface water away from the shore and generated u p w e l l i n g i n the sea i n eastern Idaho and adjacent areas i n Montana, Wyoming, and Utah. The p r e v a i l i n g wind d i r e c t i o n , determined from t r a n s p o r t d i r e c t i o n s measured on mid-Permian sand dunes i n northern Colorado and adjacent areas i n Utah and Wyoming (7_), i s c o n s i s t e n t w i t h placement of the r e g i o n w i t h i n the northern hemispheric trade-wind b e l t . The Sublett b a s i n and adjacent l a n d areas were at approximately l a t i t u d e 10° to 25° Ν as determined from the North American p l o t of the mid-Permian p o s i t i o n of the paleomagnetic pole (20). Because of the n o r t h e r l y d r i f t and counter c l o c k w i s e r o t a t i o n of the North American c o n t i n e n t to i t s present p o s i t i o n , the o r i g i n a l n o r t h e a s t e r l y paleowinds now c o i n c i d e w i t h the observed n o r t h e r l y o r i g i n of these winds seen i n the Permian rocks. Surface-water flow of Idaho was toward the southwest according to the study of Brittenham (26) and i s c o n s i s t e n t w i t h the p o s t u l a t e d o f f s h o r e flow generated by the n o r t h e a s t e r l y o f f s h o r e winds. The d i s t r i b u t i o n of o r g a n i c carbon and phosphorus d e p o s i t i o n i n d i c a t e s that the p r i n c i p a l locus of u p w e l l i n g d u r i n g d e p o s i t i o n of the Meade Peak was i n the v i c i n i t y of the Idaho-Wyoming border, and during d e p o s i t i o n of the Retort the p r i n c i p a l l o c u s of d e p o s i t i o n was i n southwestern Montana ( 3 ) . Stratigraphie Relations The i n t e r t o n g u i n g r e l a t i o n s and nomenclature a p p l i e d to the middle Permian rocks (Roadian and Wordian Stages of F u r n i s h , 21) i n the Sublett b a s i n are shown i n f i g u r e s 2 and 3. In the southwestern part of the b a s i n the Phosphoria Formation tongues i n t o the deeper b a s i n Murdock Mountain Formation, and i n the eastern part of the b a s i n the Phosphoria tongues i n t o the s h e l f carbonate d e p o s i t s of the Park C i t y Formation. P r i o r to d e p o s i t i o n of the Phosphoria, most of the Sublett b a s i n and adjacent areas were the s i t e of extensive d e p o s i t i o n of Lower Permian shallow-water carbonate sediments. In the southeastern part of the b a s i n , the carbonate rocks u n d e r l y i n g the Phosphoria are the Kaibab Limestone; and i n the northeastern p a r t of the b a s i n , where these carbonates are g e n e r a l l y sandy and intertongue w i t h sandstone s t r a t a , they are c a l l e d the Grandeur Member of the Park C i t y Formation. Rocks e q u i v a l e n t to the Grandeur i n the northern p a r t of the b a s i n i n southwestern Montana are dominantly sandstone that are included i n the Quadrant Sandstone and those i n



Figure 3. Cross-section A-A' showing generalizedfacies relations and nomenclature of middle Permian strata across Sublett basinfrom Cassia Mountains, Idaho to central Wyoming. (Section location shown in Figure I.)




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the southeastern s e c t o r are part of the Weber Sandstone i n northeastern Utah, The Phosphoria and Park C i t y Formations were deposited upon carbonate and sandstone s t r a t a of the Kaibab, Grandeur, Quadrant, and Weber i n two c y c l i c sequences. The lower sequence comprises, i n ascending o r d e r , the Meade Peak Phosphatic Shale Member and the Rex Chert Member of the Phosphoria Formation, and the Franson Member of the Park C i t y Formation. The upper sequence comprises the Retort Phosphatic Shale Member and the T o s i Chert Member of the Phosphoria and the Ervay Limestone Member of the Park C i t y . The members i n the upper sequence are not as t h i c k as those i n the lower sequence, and the depocenter of the carbonaceous mudstone of the Retort i s l o c a t e d about 110 km north-northwest of the depocenter of s i m i l a r rocks of the Meade Peak i n the lower sequence. The n o r t h e r l y s h i f t of the depocenter was accompanied by a s h i f t of both the southern and northern s h o r e l i n e s , the d i s t r i b u t i o n of the l i t h o f a c i e s , and a corresponding s h i f t of the center of maximum average organic carbon and phosphorite d e p o s i t i o n of the Meade Peak and Retort Members. This s h i f t probably resulted from regional tilting away from the Transcontinental arch. The d e p o s i t i o n a l sequence w i t h i n the two phosphatic shale members comprise s a p r o p e l i c sediments that i n c l u d e p e l o i d a l phosphorite and subordinate phosphatic mudstone near the base, o r g a n i c - c a r b o n - r i c h mudstone w i t h i n the c e n t r a l p a r t , and p e l o i d a l phosphorite and phosphatic mudstone i n the upper p a r t . Shelfward, the bases of the shale members are i n sharp contact w i t h the u n d e r l y i n g rocks, but the contacts seem to be s h a r p l y t r a n s i t i o n a l i n b a s i n a l areas. The upper contacts are t r a n s i t i o n a l upward i n t o the cherty beds of the o v e r l y i n g member i n a l l areas. Thicknesses Isopachs of the Meade Peak and the R e t o r t are shown i n f i g u r e 4. Maximum thicknesses of s a p r o p e l i c mudstone and phosphorite composing the Meade Peak Member i s about 60 m i n a b e l t that approximately c o i n c i d e s w i t h the Wasatch and Bear River Ranges from the v i c i n i t y of Qgden, Utah to near P o c a t e l l o , Idaho ( f i g . 4). The t h i c k e s t s e c t i o n s of Meade Peak l i e w i t h i n the W i l l a r d Bannock t h r u s t p l a t e so that s p a t i a l r e l a t i o n s are complicated by thrust displacement subsequent to Permian deposition. N e v e r t h e l e s s , i t i s evident that sapropel accumulation was g r e a t e r near the geographical center of the Sublett b a s i n , r a t h e r than c o i n c i d i n g w i t h the c e n t e r ( s ) of subsidence where the Park C i t y Group i s t h i c k e s t i n the western p a r t of the b a s i n adjacent to the Humboldt p o s i t i v e b e l t i n n o r t h - c e n t r a l Nevada. The isopach map of the Meade Peak ( f i g . 4) shows approximately uniform t h i n n i n g of the member c e n t r i f u g a l l y from the c e n t r a l p a r t of the b a s i n along the Ogden to P o c a t e l l o l i n e ; t h i s i s e s p e c i a l l y evident i f p a l i n s p a s t i c adjustments are considered. Sapropel accumulation



Figure 4. Isopach map of Meade Peak (left) and Retort (right) Members of the Phosphoria Formation; contour interval is 10 m. Principal overthrust faults of the Sevier thrust belt indicated by barbed line; isopachs andfaults are dashed where uncertain.




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seems to have been s y n d e p o s i t i o n a l l y d i s p l a c e d eastward from the depocenter of the Sublett basin i n northeastern Nevada by the i n f l u x of terrigenous d e t r i t u s from the Humboldt highland to the west. The Meade Peak t h i n s to the n o r t h , e a s t , and south away from the area of maximum sapropel accumulation and towards areas of s h o a l i n g , which were unfavorable environments f o r p r e s e r v a t i o n of organic matter i n the oxygenated water near the shore of the Phosphoria sea. The Retort Member i s t h i c k e s t i n southwestern Montana. The Retort c l o s e l y resembles the Meade Peak except t h a t i t s t h i c k n e s s i s about one-half that of the Meade Peak and i t s s h o r e l i n e s and apparent depocenter are d i s p l a c e d northward. The R e t o r t , l i k e the Meade Peak, i s b e l i e v e d to have been deposited along a b e l t that extended southward i n t o eastern Idaho from the apparent depocenter i n southwestern Montana. However, the Upper Permian sequence has been eroded i n most of eastern Idaho p r i o r to d e p o s i t i o n of T r i a s s i c s t r a t a (22) and has l e f t the o r i g i n a l extent of the Retort unknown. Organic Carbon Organic-carbon content i n the Meade Peak ( f i g . 5) i s g r e a t e s t , averaging about 9 weight percent, near the Wyoming border east of P o c a t e l l o , Idaho ( 3 ) , and i s o f f s e t northeastward from the c e n t r a l a x i s of the b a s i n onto the f l a n k of the area of t h i c k e s t accumulation of organic-carbon-rich mudstone. This o f f s e t p o s i t i o n of the maximum organic^carbon c o n c e n t r a t i o n i s b e l i e v e d to approximately c o i n c i d e w i t h an area of upwelling marine currents from out of the deeper p a r t s of the S u b l e t t b a s i n onto the submarine slope of f r i n g i n g b a r r i e r i s l a n d and carbonate bank d e p o s i t s . The locus of maximum organic-carbon d e p o s i t i o n i n the Meade Peak ( f i g . 5) approximately c o i n c i d e s w i t h that of maximum phosphorus concentration ( f i g . 6 ) . However, phosphorite and s a p r o p e l i t e were deposited over the e n t i r e S u b l e t t b a s i n , p o s s i b l y because of the production of an e x c e p t i o n a l l y abundant, probably dominantly phytoplanktonic, biomass and i t s widespread d i s p e r s i o n throughout the Phosphoria s e a . The average organic-carbon content of the Meade Peak i n the Sublett b a s i n , the average of 285 samples from 40 l o c a l i t i e s , i s c a l c u l a t e d to be about 2.4 percent, and some beds c o n t a i n as much as 30 percent organic carbon by weight. These are values s u b s t a n t i a l l y higher than the 1.02 percent average determined f o r marine l i t t o r a l mudstone by Ronov (23 p. 11). Most of the s t r a t a w i t h i n the Meade Peak c o n t a i n i n excess of the 0.5 percent organic carbon considered necessary f o r an adequate petroleum source rock. Organic-carbon content i n the Retort i s g r e a t e s t , averaging about 10 weight percent, near D i l l o n , Mont. ( f i g . 4 ) . The center of maximum organic carbon d e p o s i t i o n i n the Retort ( f i g . 5) approximately coincides with that of maximum phosphorus



Figure 5. Organic-carbon distribution in Meade Peak (left) and Retort (right) Members ofthe Phosphoria Formation shown by isograms of average weight percent.


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c o n c e n t r a t i o n ( f i g . 6 ) . The average organic-carbon content of the R e t o r t , the average of 82 samples from 22 l o c a l i t i e s , i s about 4.9 percent, double the average f o r the Meade Peak. The d i f f e r e n c e seems a t t r i b u t a b l e to a s t a t i s t i c a l b i a s created by t h e absence by e r o s i o n of leaner s a p r o p e l i c mudstones of the Retort from l a r g e areas to the south of the depocenter i n southwestern Montana, r a t h e r than to any s i g n i f i c a n t geochemical d i f f e r e n c e between the Retort and the Meade Peak. The abundant and u b i q u i t o u s organic matter i n the Phosphoria sea may have been produced i n a way s i m i l a r t o t h a t i n Walvis Bay on the southwest A f r i c a n coast, which Brongersma-Sanders (19) explained by the r e l a t i o n s h i p of u p w e l l i n g , l a r g e biomass production, and the accumulation of b i o l o g i c a l remains on the sea f l o o r as s a p r o p e l i c ooze. Phosphorite Deposits of phosphorite i n the Meade Peak Member are most abundant i n eastern Idaho and adjacent areas i n Wyoming and Utah where they a r e mined. A b e l t of phosphorus ( f i g . 6) p r o j e c t s southeastward from eastern Idaho into the e a s t e r n U i n t a Mountains. The average phosphorus values shown i n f i g u r e 6 are an index t o the abundance of phosphorite along t h i s b e l t . Similarly, phosphorus i n the Retort ( f i g . 6) i s an index to the phosphorite, which i s most abundant i n southwestern Montana i n t h i s member. Discrepancy between the centers of d i s t r i b u t i o n of the maximum organic carbon and t h a t f o r the maximum phosphorus a r e b e l i e v e d t o be owing t o mechanical s o r t i n g . According t o Burnett (24), recent phosphorite d e p o s i t s o f f the coast of Peru and C h i l e probably form d i a g e n e t i c a l l y w i t h i n anoxic, h i g h l y organic sediments, and the a p a t i t e subsequently i s concentrated by winnowing and reworking i n response t o changes i n water c u r r e n t s caused by e u s t a t i c seal e v e l changes or by tectonism. The o r i g i n of these Permian phosphorites and the approximate, but not p r e c i s e , coincidence of depocenters f o r organic carbon and phosphorus seems best e x p l a i n e d by s i m i l a r winnowing and reworking. Economically mined beds of phosphorite i n the Meade Peak Member i n eastern Idaho and adjacent areas i n Utah and Wyoming approximately c o i n c i d e w i t h the l o c a t i o n of the Bannock h i g h l a n d ( f i g . 1 ) . U p l i f t of t h i s highland e a r l y i n the Pennsylvanian P e r i o d occurred i n the area (25). S l i g h t u p l i f t may have r e c u r r e d i n about the same area during the time of d e p o s i t i o n of the Meade Peak t o form a broad r i d g e submerged beneath the Phosphoria s e a where winnowing could have been a c t i v e . A d e t e c t a b l e i r r e g u l a r i t y i n the l e v e l of the sea f l o o r c o i n c i d e n t w i t h the Bannock h i g h l a n d i s not evident i n thickness v a r i a t i o n s of the Meade Peak Member shown i n f i g u r e 3A; but p o s s i b l y the 10-m isopach i n t e r v a l i s t o o coarse to r e f l e c t minor s h o a l i n g i n that area. However, r e c u r r e n t u p l i f t i n approximately the same area as the Pennsylvanian u p l i f t i s i n d i c a t e d by s h o a l i n g of the Phosphoria sea i n southeastern



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Idaho as evidenced by bioherms i n the Rex Chert Member of the Phosphoria Formation directly above the Meade Peak (26)» Recurrent u p l i f t and e r o s i o n of Permian s t r a t a c o i n c i d e n t w i t h the area of the Bannock highland a l s o took place p r i o r to T r i a s s i c d e p o s i t i o n ( 3 , 22) as i n d i c a t e d by the absence of R e t o r t ( f i g . 4B) and younger Permian rocks i n parts of eastern Idaho and some adjacent areas. Most phosphorite i n eastern Idaho has a p e l o i d a l f a b r i c and i s composed of a packstone of s p h e r i c a l to s l i g h t l y o b l a t e g r a i n s that range mostly from about 0.2 to 2 mm i n diameter. Many p e l o i d s are nucleated around a s i l t - s i z e quartz p a r t i c l e , although rod-shaped to round phosphatized f e c a l p e l l e t s a r e common and bone fragments form the n u c l e i i n some p e l o i d s . The t e x t u r e i n the body of many p e l o i d s i s non-structured and appears to comprise phosphatized amorphous, dark-brown organic matter surrounded by a t h i n s h e l l of m i c r o - c r y s t a l l i n e a p a t i t e . Many other p e l o i d s comprise c o n c e n t r i c a l l y banded laminae of a l t e r n a t e l y dark- and light-brown m i c r o - c r y s t a l l i n e a p a t i t e and some i n c l u d e amorphous organic matter. Phosphatized s k e l e t a l remains that i n c l u d e s p i c u l e s , s h e l l fragments, and bones occur i n some t h i n s t r a t a , but are not common. I n t e r g r a n u l a r spaces a r e f i l l e d c h i e f l y by sparry apatite, sparry calcite, dolomite, or additional phosphatized and amorphous organic matter. Clay minerals a r e common, but not abundant i n many of these p e l o i d a l phosphorites. The f a b r i c of the phosphorites deposited i n northeastern Utah d i f f e r s from the f a b r i c of equivalent phosphorite i n c e n t r a l Idaho. Chief d i f f e r e n c e s a r e b e l i e v e d to be due t o the e f f e c t of d i f f e r e n t water depths and current energies i n the two areas. The northeastern Utah phosphorites were deposited near the margin of the S u b l e t t b a s i n where there presumably would have been shallower water and higher energy currents i n c o n t r a s t to the apparent d e p o s i t i o n i n a low-energy environment f a r t h e r from the b a s i n margin i n eastern Idaho. A greater v a r i e t y of g r a i n s i z e s , l i t h o l o g i e components, and textures occur i n northeastern Utah. Fragmented p e l o i d s and other angular to sub-rounded c l a s t s a r e abundant components of the phosphorite beds, as i s phosphatized skeletal debris. O o l i t i c phosphorite g r a i n s comprised o f c o n c e n t r i c l a y e r s of m i c r o - c r y s t a l l i n e a p a t i t e are dominant constituents, and amorphous organic matter seems less conspicuous. P e l o i d s commonly are as much as 5 mm i n diameter and a p a t i t e nodules are as much as 2 cm i n diameter. S i l t - and sands i z e quartz grains occur i n many beds and are the dominant components i n some of the beds that a r e i n t e r s t r a t i f i e d w i t h the phosphorites. Chert fragments are l o c a l l y abundant, a l s o , and phosphorite nodules, which are r a r e i n eastern Idaho, a r e common in some beds i n the presumed shallower water deposits i n northeastern Utah.


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Trace Metals Traces of many metals, some of unusually high c o n c e n t r a t i o n , occur i n the carbonaceous shale and phosphorite beds of t h e Phosphoria. These trace elements have been determined s e m i q u a n t i t a t i v e l y by emission spectrographic analyses. The a n a l y t i c a l data f o r the metals have been presented p r i n c i p a l l y by Gulbrandsen (9) and by Maughan ( 1 1 ) . The l o c i of the average concentration of s e v e r a l metals i n the carbonaceous shale members a l s o approximate the l o c i o f organic carbon and phosphorus. The values f o r these metals a r e s i g n i f i c a n t l y higher than the average f o r marine shales as reported by Rankama and Sahama (27_ p. 226). The d i s t r i b u t i o n of s i l v e r i n the Meade Peak and the Retort i s shown i n f i g u r e 6, vanadium i n f i g u r e 7, and lanthanum i n f i g u r e 8. I t seems probable that some of the t r a c e metals were concentrated from the sea water by the organisms l i v i n g i n the Phosphoria sea. Other metals were adsorbed onto the r e s i d u a l organic matter e i t h e r d u r i n g i t s excursion to the sea f l o o r , or during the residence of the organic matter at or near the d e p o s i t i o n a l i n t e r f a c e . Yet other m e t a l l i c concentrations r e s u l t e d from chemical s u b s t i t u t i o n s w i t h i n the carbonate f l u o r a p a t i t e or w i t h i n other m i n e r a l s , especially the c l a y s , during the e a r l y diagenesis of the s a p r o p e l i c mud on the sea f l o o r . The coincidence of the m e t a l l i c concentrations and the organic carbon i n the Retort and the Meade Peak a r e b e l i e v e d to confirm the i n d i c a t e d l o c a t i o n of centers o f u p w e l l i n g i n southwestern Montana and i n the v i c i n i t y of the Idaho-Wyoming border. H i t e (28) has described a mechanism f o r concentrating phosphorus and other elements and i n t r o d u c i n g them i n t o t h e Phosphoria sea by r e f l u x c i r c u l a t i o n from the adjacent h y p e r s a l i n e lagoon i n eastern Wyoming. This mechanism may have been an important f a c t o r i n p r o v i d i n g the t r o p h i c requirements t o s u s t a i n the l a r g e biomass that c o n t r i b u t e d the organic matter t o the f l o o r of the S u b l e t t basin. On the other hand, volcanism i n the probable i s l a n d a r c a t the northwestern margin of the b a s i n may have been the primary source of many of the chemical elements that enriched the Phosphoria sea, a concept that hearkens back t o e a r l i e r s p e c u l a t i o n s that the abundant s i l i c a i n the adjacent cherty members, which intertongue w i t h the phosphatic shale members, may have o r i g i n a t e d from Permian volcanism i n western Idaho (1_ p. 371-372; 2_ p. 27). The i n f l u x of v o l c a n i c ash i n t o the Phosphoria sea i s i n d i c a t e d by the data of Gulbrandsen ( 2 9 ) , who i d e n t i f i e d Buddingtonite, an ammonium f e l d s p a r m i n e r a l formed by the i n t e r a c t i o n of v o l c a n i c g l a s s and organic matter, i n samples from the Meade Peak and the Retort Members. The c o n c e n t r a t i o n of t r a c e metals by v o l c a n i c i n p u t , by the r e f l u x of h y p e r s a l i n e waters, or by other e x t r a o r d i n a r y m e t a l l i c i o n enrichment of the Phosphoria sea a r e not r e q u i r e d according t o data and i n t e r p r e t a t i o n s given by Calvert (30 p. 201-212).



Figure 7. Silver distribution in Meade Peak (left) and Retort (right) Members of the Phosphoria Formation shown by isograms, in parts per million. (Isogram dashed where uncertain.)


C/3

r m

>

C/5

F

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χ m

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Figure 8. Vanadium distribution in Meade Peak (left) and Retort (right) Members of the Phosphoria Formation shown by isograms, in parts per million.


to



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Phosphorites occur i n o r g a n i c - r i c h muds deposited on the s h e l f o f f southwest A f r i c a , which a r e a l s o enriched i n c e r t a i n minor metals that are not of terrigenous o r i g i n (30 p. 209-210)· The metals i n these sediments are w i t h i n the ranges of the metal content of marine plankton (30 p. 212), and the organic m a t e r i a l i n the bottom sediments probably are enriched i n these metals by adsorption. Brongersma-Sanders (31 p. 233) i n d i c a t e d that t r a c e metals g e n e r a l l y are not concentrated i n the sediments except " . . . i n places of highest organic p r o d u c t i v i t y , where the number of concentrating organisms i s high, e.g. i n areas of u p w e l l i n g water." For p r e s e r v a t i o n of t h i s organic matter and i t s trace metals, anoxic bottom c o n d i t i o n s are necessary; but the accumulation and decay of dead plankton s e t t l i n g from the f e r t i l e surface waters can q u i c k l y lead to de^oxygénâtion of the bottom waters and the u n d e r l y i n g sediment (31 p. 235). Thus, the c o n c e n t r a t i o n and d i s t r i b u t i o n of the elements i n the Meade Peak and Retort shown i n f i g u r e s 5 to 9 i n d i c a t e areas of high accumulation of organic matter i n the S u b l e t t b a s i n t h a t probably were areas of u p w e l l i n g and l a r g e b i o l o g i c a l p r o d u c t i v i t y i n the Phosphoria sea. The emission spectrographic data (11) suggest that a c l o s e a s s o c i a t i o n of organic carbon to s i l v e r e x i s t s (compare f i g s . 5 and 7 ) , as w e l l as to chromium, molybdenum, n i c k e l , t i t a n i u m , vanadium ( f i g . 8 ) , and z i n c . The spectrographic analyses f o r phosphorus show that a c l o s e a s s o c i a t i o n of the maximum f o r t h i s element to maximum organic carbon e x i s t s ; but that near-maximum values of phosphorus extend from eastern Idaho i n t o northeastern Utah, which i s an area of comparatively low organic carbon (compare f i g s . 5 and 6 ) . Phosphorus seems c l o s e l y a s s o c i a t e d w i t h copper, lanthanum ( f i g . 9 ) , neodymium, s t r o n t i u m , y t t r i u m , and ytterbium. C o r r e l a t i o n analyses and other proposed s t u d i e s , e s p e c i a l l y i n t o the c l a y mineralogy, may enable us to b e t t e r understand these r e l a t i o n s h i p s and to determine the o r i g i n s of these a s s o c i a t i o n s . Petroleum

Generation

Petroleum generation from the Phosphoria Formation has been i n v e s t i g a t e d and a t o t a l y i e l d of 30.7 χ 1 0 m e t r i c tons i s estimated by Claypool and others (A_ p. 118). The bulk of the Phosphoria o i l seems to have been generated from the mudstone s t r a t a r a t h e r than from the phosphorite beds as suggested by Powell and others (32). Graphic comparisons ( f i g . 10) of organic carbon, phosphorite, bitumen, and hydrocarbon from the data of Claypool and others (4_ p. 105) and from Maughan (3) show an expectable r e l a t i o n between organic carbon and bitumen, but l i t t l e r e l a t i o n s h i p between the other chemical f a c t o r s i n these r o c k s , i n c l u d i n g phosphorus. The graph shows that organic carbon, hydrocarbon, and bitumen are common to abundant i n mudstones having a low phosphorous ( · ι··

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00

0 0

15

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Figure 10. Plot comparing organic carbon and bitumen (left), and phosphorus to organic carbon, bitumen, and hydrocarbons (right) in samples from Meade Peak and Retort Members of the Phosphoria Formation. Values in percent for organic carbon and phosphorus are from Maughan (11); bitumen and hydrocarbons from Claypool et al. (4).

4.0

X

•


4

3


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components are comparatively low i n the phosphorites (>7% Ρ)· Hydrocarbons and phosphorite have a common o r i g i n i n t h e sapropel. The anoxic marine d e p o s i t i o n a l environment f a v o r a b l e f o r the accumulation of o r g a n i c ^ c a r b o n - r i c h mudstone was f a v o r a b l e f o r the formation of phosphorites and f o r n i t r o g e n - r i c h o i l s . However, the hydrocarbons are c h i e f l y the product of catagenesis that i s dependent mostly on b u r i a l and the consequent i n c r e a s e of the ambient temperature, whereas phosphorite seems t o be the product of penecontemporaneous diagenesis that i s c h i e f l y dependent on anaerobic decomposition of the organic matter, minéralogie r e c o n s t i t u t i o n , and mechanical s o r t i n g . Oil that has been discovered and produced from t h e Pennsylvanian Tensleep Sandstone ( f i g . 11), the probable Lower Permian part of the Weber Sandstone i n northwestern Colorado, and the middle Permian Park C i t y Formation i n c e n t r a l Wyoming probably has been d e r i v e d from the Phosphoria Formation. O i l i n the Lower Permian upper member of the Minnelusa Formation i n northeastern Wyoming may have migrated i n t o these r e s e r v o i r s from t h e Phosphoria source beds, but carbonaceous beds i n the middle member of the Minnelusa are a more l i k e l y source. Cheney and Sheldon (33) speculated that petroleum o r i g i n a t e d i n the organic-carbonr i c h shale beds of the Phosphoria Formation and that the o i l migrated eastward and was trapped i n r e s e r v o i r s i n the e q u i v a l e n t carbonate rocks of the Park City Formation. Sheldon (34) a l s o r e l a t e d o i l accumulation i n upper P a l e o z o i c rocks to i t s probable o r i g i n i n shales of the Phosphoria and suggested t h a t m i g r a t i o n from eastern Idaho and western Wyoming occurred i n response to accumulating overburden and p r o g r e s s i v e l y e a s t w a r d - s h i f t i n g tectonic forces. He i n d i c a t e d that o i l generated i n the Phosphoria i n eastern Idaho began eastward m i g r a t i o n e a r l y i n Jurassic time. O i l generation and m i g r a t i o n developed i n c r e a s i n g l y f a r t h e r eastward as the T r i a s s i c , J u r a s s i c , and Cretaceous sedimentary l o a d i n g of the Permian s t r a t a increased p r o g r e s s i v e l y eastward i n western Wyoming; but t e c t o n i c b a r r i e r s t h a t developed w i t h the Laramide orogeny prevented f u r t h e r longd i s t a n c e m i g r a t i o n i n M a e s t r i c h t i a n and l a t e r times. Data presented by Stone (35) s u b s t a n t i a t e d the hypothesis that most of the hydrocarbons i n P a l e o z o i c and T r i a s s i c rocks of the Bighorn B a s i n i n n o r t h - c e n t r a l Wyoming came from the carbonaceous shale beds of the Phosphoria. The r e l a t i o n o f petroleum produced from r e s e r v o i r s i n the Pennsylvanian Tensleep Sandstone t o the black shales i n the Phosphoria Formation and t o the i n f e r r e d maximum depth of b u r i a l of these rocks i s shown i n f i g u r e 11. In eastern Idaho, the c r i t i c a l depth of b u r i a l occurred as e a r l y as Late T r i a s s i c , and r e g i o n a l l y the maximum depth of b u r i a l of the Permian rocks i s i n f e r r e d t o have been a t the end of the Cretaceous P e r i o d , although sediments continued to accumulate i n l o c a l , intermontane basins through the Paleocene and i n t o e a r l y Eocene time (4_p. 101-104).




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Figure 11. Relation of oil reservoirs (solid patterns) in Pennsylvanian Tensleep Sandstone to carbonaceous shale beds of the Permian Phosphoria Formation and to inferred maximum depth of burial (thickness of Mesozoic rocks, in kilometers) shown by solid lines and supplementary 2.5- and 3.5-km thickness shown by dashed line.


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Catagenetic conversion of kerogen t o hydrocarbons has taken place throughout most of the Sublett b a s i n . Hydrocarbon generation has taken place i n most of the region where b u r i a l has been i n excess of about 2 km. B u r i a l between about 2 km and 5 km as shown by Claypool and others (4_ f i g . 7) and the temperature a t these depths has brought these rocks i n t o the " o i l g e n e r a t i o n window." A t b u r i a l depths g r e a t e r than about 5 km, ambient temperatures have l e d t o the degradation of the hydrocarbons and has exhausted the p o t e n t i a l f o r f u r t h e r c a t a g e n e t i c r e a c t i o n and hydrocarbon generation from the kerogen. At l e s s than about 3 km of b u r i a l , the p o t e n t i a l f o r hydrocarbon generation from the kerogen remains, and where the organic carbon content i s h i g h , these thermally immature rocks are c l a s s i f i e d as o i l s h a l e s . O i l Shales The R e t o r t Member i n some parts of southwestern Montana i s an o i l shale r i c h i n organic carbon, i t i s thermally immature, and the shale has the p o t e n t i a l f o r the s y n t h e t i c generation of hydrocarbons. The r e c o g n i t i o n of o i l s h a l e i n the Phosphoria was f i r s t noted i n a report by Bowen (36) and i n a more complete report the f o l l o w i n g year by Condit ( 5 ) . Attempts t o p y r o l i t i c a l l y produce o i l from the Retort Member near D i l l o n , Mont., between about 1915 and 1925 were moderately s u c c e s s f u l , but the expense of mining and processing was not competetive w i t h more cheaply produced crude o i l from w e l l s . A 1.7-m-thick bed i s reported (5_p. 23) to have y i e l d e d 87.5 L/metric ton of o i l , and a y i e l d of 100 L/ton had been obtained i n the e a r l i e r study by Bowen (36 p. 318). I n contrast to these moderately high y i e l d s from southwestern Montana, samples from g e n e t i c a l l y and l i t h o l o g i c a l l y s i m i l a r but p r e v i o u s l y deeply buried and p r e s e n t l y thermally overmature phosphatic s h a l e beds of the Meade Peak Member i n Idaho and Wyoming, were reported (5_ p. 31-32) to y i e l d only t r a c e s of oil. S o l i d - s t a t e nuclear magnetic resonance spectra of Phosphoria shales (37) i n d i c a t e the residence of h i g h e r amounts of a l i p h a t i c carbon i n the thermally immature samples from southwestern Montana compared to thermally mature samples from Idaho. S i m i l a r l y , gas chromatographic analyses and the r a t i o of hydrocarbon t o carbon (4) i n d i c a t e thermal immaturity and the p o t e n t i a l f o r hydrocarbon generation i n the southwestern Montana o i l shales i n c o n t r a s t to the depleted hydrocarbon-producing p o t e n t i a l of kerogenic s h a l e s i n other areas. The o i l shale i n southwestern Montana occurs i n s t r u c t u r a l l y complex f o l d e d and f a u l t e d mountains. Mining and p r o c e s s i n g o f large quantities of these shales would require difficult subsurface methods. Economical e x p l o i t a t i o n w i l l depend upon the development of e x t r a c t i o n technology that may r e q u i r e the recovery of the m u l t i p l e resources of hydrocarbons, phosphorous, and t h e other metals.


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Acknowledgments J . Stewart Williams provided me an introduction to the Phosphoria Formation in both classroom and field. E. D. McKee, S. S. Oriel and others involved in the preparation of the U.S. Geological Survey paleotectonic maps provided encouragement and concepts that have led to an understanding of the paleogeography of these rocks. Discussions with R. P. Sheldon have contributed much of the insight and concepts relating to upwelling and to the regional stratigraphie relations of these rocks. R. F. Wilson cooperated in the determination of the prevailing paleowind directions by studies of sand-dune cross-beds. Chemical analyses (the carbon values by V. E. Shaw and T. L. Yager and the semiquantitative emmission spectrographic determinations of elements other than carbon by J. C. Hamilton and L. A. Bradley) were done in the Analytical Laboratories of the U.S. Geological Survey at Denver, Colorado. The assistance in both field and office of many other individuals is also acknowledged and is greatly appreciated. Literature Cited 1. Mansfield, G. R. U.S. Geol. Surv. Prof. Paper 152 1927. 2. McKelvey, V. E . ; and others. U.S. Geol. Surv. Prof. Paper 313-A 1959. 3. Maughan, Ε. Κ., in "27th Ann. Field Conf. Guidebook"; Wyo. Geol. Assoc.: Casper, Wyo., 1975; p. 107-115. 4. Claypool, G. E . ; Love, A. H.; Maughan, Ε. K. Am. Assoc. Petrol. Geol. Bull. 1978, 62, 98-120. 5. Condit, D. D. U.S. Geol. Surv. Bull. 711-B 1919, p. 15-40. 6. Sheldon, R. P.; Maughan, E. K.; Cressman, E. R. in "Paleotectonic maps of the Permian System" U.S. Geol. Surv. Misc. Geol. Inv. Map I-450, 1967, text p. 48-54; also in "Anatomy of the western phosphate field"; 15th Annual Field Conference 1968, Intermtn. Assoc. Geol., p. 1-13. 7. Maughan, Ε. K. in "2nd Symposium on salt, v. 1, Geology, geochemistry, and mining"; North. Ohio Geol. Soc.: 1966; p. 35-47. 8. Maughan, Ε. K. in "Great Basin Guidebook"; Rocky Mtn. Assoc. Geol., Utah Geol. Assoc.: 1979; p. 523-530. 9. Gulbrandsen, R. A. U.S. Geol. Surv. Open-File Report 75-554 1975. 10. Gulbrandsen, R. A. U.S. Geol. Surv. Open-File Report 79-369 1979. 11. Maughan, Ε. K. U.S. Geol. Surv. Open-File Report 76-577 1976. 12. Maughan, Ε. K. in "Géologie comparée des gisements de phosphates et de pétrole, colloque international, 1979"; Bureau de Recherches Géologiques et Minières Document 24: Orleans, France, 1980, p. 63-91.



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