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Geochemical characteristics of the Paleogene and Neogene saline lacustrine source rocks in the western Qaidam Basin, northwestern China Chenglin Liu, Haohan Li, Xu Zhang, Shijing Zheng, Lin Zhang, Zeqing Guo, Jun Liu, Jixian Tian, Ying Zhang, and Xu Zeng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00269 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016
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Geochemical characteristics of the Paleogene and Neogene saline lacustrine source rocks in the western Qaidam Basin, northwestern China Chenglin Liu1,2, Haohan Li1, Xu Zhang2, Shijing Zheng1, Lin Zhang3, Zeqing Guo3, Jun Liu1, Jixian Tian3, Ying Zhang3, Xu Zeng3 1. State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing, 102249, China. 2. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, 100081, China. 3. Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina, Lanfang, Hebei Province, 065007, China.
ABSTRACT Based on salinity and organic geochemical analyses of mudstones, this paper analyzed the relationship between salinity and sedimentary facies of different strata, studied influences of salinity on organic matter richness, types and thermal maturities of source rocks and built the organic enrichment accumulation model for the Paleogene and Neogene saline lakes of the western Qaidam Basin. In the western Qaidam Basin, saline lake depocenters and high salinity areas migrated eastward, and chloride ion (Cl-) concentration of source rocks showed an increasing trend from the Paleocene to Pliocene. The relationship between organic matter content and salinity of the Paleocene and Pliocene source rocks shows a three-interval model: both TOC (total organic carbon) content and chloroform bitumen content have low values at the low salinity stage, firstly increase to the peak and then decrease to the lowest values at the medium salinity stage, and sustain the lowest values at the super saline stage because of the interaction between organic productivity and reduction-oxidation in the saline lake of the western Qaidam Basin. The relationship between organic matter type and salinity of these source rocks suggests that the kerogen increasingly corresponds to a sapropelic and planktonic origin with increasing salinity. The organic matter enrichment model of the Paleogene and Neogene saline lakes of the western Qaidam Basin showed that saline lakes had high productivity and reducing conditions favorable for organic matters accumulation and preservation at their middle evolution stage. Through improving their apparent activation energy, salinity slowed down thermal evolution of source rocks, making the Paleogene and Neogene source rocks at low mature to immature stage in the western Qaidam Basin.
Key Words: Salinity; Chloride ion concentration; Organic matter; Pyrolysis; Kerogen.
1. INTRODUCTION There are multiple Mesozoic and Cenozoic fresh-brackish and saline-hypersaline lacustrine basins in China.1,
2
As to a saline lake, the relatively closed sedimentary system could form a
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reducing environment favorable for organism accumulation and preservation.3 Saline lakes are characterized by a low diversity but high abundance of organisms and by interbeded evaporites and source rocks.4-6 In saline lacustrine basins, source rocks have features of high organic matter content and elevated ratios of soluble organic matter to TOC content.7-18 Through catalytic or reversely catalytic reactions, clay, carbonate and salt minerals have effects on thermal evolution of source rocks, which could be simulated by open-system, closed-system and half open-system programmed-temperature pyrolysis experiments.19-33 The purposes of this paper were to determine salinity geohistorical and spatial distributions, analyze the relationship between salinity and sedimentary facies of different strata, study the effects of formation salinity on organic matter content, types and thermal maturities of source rocks, and evaluate the geochemical characteristics of the Paleogene and Neogene source rocks in the western Qaidam Basin.
2. GEOLOGIC SETTING Situated in the north of the Qinghai-Tibetan plateau, the Qaidam Basin is a Mesozoic and Cenozoic nonmarine and Paleozoic marine petroliferous basin, surrounded by the Kulun, A-erh-chin and Qilian mountains.34, 35 With an area of 121,000 km2, it is divided into three geological units, i.e., the western depression, northern faulted block zone, and eastern depression (Figure 1).2, 36-38 The basement is made up of Precambrian metamorphic rocks. In the Paleogene and Neogene, saline lakes developed in the western Qaidam Basin. Paleogene includes Lulehe Formation (E1+2) and Xiaganchaigou Formation (E3), and Neogene includes Shangganchaigou Formation (N1), Xiayoushashan Formation (N21), Shangyoushashan Formation (N22) and Shizigou Formation (N23) (Figure 2). Source rocks are mainly dark or dark grey mudstones, calcareous mudstones and silty mudstones developed from the Paleocene to Miocene, especially in the upper segment of Xiaganchaigou Formation (E32) and Shangganchaigou Formation (N1). Main reservoirs include porous sandstones and siltstones, solution porous algae limestones, and fractured mudstones and muddy limestones developed from Xiaganchaigou Formation (E3) to Shizigou Formation (N23). The source-reservoir-cap assemblages include the middle Miocene shale source rocks-the lower Miocene sandstone reservoirs-the Neogene shale cap rocks, the Oligocene shale source rocks-the Oligocene sandstone and limestone reservoirs- the Neogene shale cap rocks, and the Oligocene and lower Miocene shale source rocks-the middle and upper Miocene sandstone and fractured mudstone and muddy limestone reservoirs-the Pliocene shale cap rocks (Figure 2).39-43
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Figure 1. Structural unit divisions and sample collection locations in the Qaidam Basin.
Figure 2. The Paleogene and Neogene formation divisions, lithology, and source-reservoir-cap assemblages in the western Qaidam Basin.
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Since the first oil field was discovered in the late 1950s, there have been 15 oil fields and 6 gas fields found in the western Qaidam Basin by the end of 2012, including the Shizigou and Nanyishan oil fields and the Xiaoliangshan and Kaitemilike gas fields. Oil from these discovered fields has the geochemical features of dominantly low-maturity, the presence of pregnane and C30 steranes, and uncommonly heavy carbon isotopes for saturated and aromatic hydrocarbon fractions. Natural gas from these fields displays a high dryness coefficient and heavy carbon isotopes for methane, ethane and propane. These oil and gas geochemical features are linked to the Paleogene and Neogene saline lacustrine environments developed in the western depression.16, 38-40, 44-47
3. SAMPLES AND ANALYTICAL METHODS Samples were collected from wells in the western Qaidam Basin and their positions were showed in Figure 1. The lithologies of these samples are mainly mudstones, calcareous mudstones and silty mudstones distributed from the Paleocene to Pliocene. Cl- concentration, TOC content, chloroform bitumen content and soluble organic component tests, and Rock-Eval pyrolysis of source rocks were conducted by the State Key Laboratory of Petroleum Resource and Prospecting of China University of Petroleum. Silver nitrate titration was used for testing Cl- concentration. The main test processes included diluting core sample powders with distilled water and titrating the powder solution with a standard solution of silver nitrate. Potassium chromate was used as an indicator. When the solution turned brick red, it was the end point of titration. The Cl- concentration was calculated according to the consumption of standard silver nitrate solution. Dry-combustion was adopted for testing TOC content. After crushed and washed by hydrochloric acid solution to eliminate the interference of carbonates, the samples were heated to 900℃ with oxygen-rich carrier gas and organic carbon in samples was completely oxidized into carbon dioxide. TOC contents of the samples were calculated based on the volume of generated carbon dioxide. These tests followed the State Standard of the People's Republic of China, Determination of Total Organic Carbon Contents in Sedimentary Rock (GB/T 19145-2003). Chloroform bitumen content was prepared by using the Soxhlet extraction method following the Standard oil and natural gas of the People's Republic of China, Determination of Chloroform Bitumen Contents from Rocks by Chloroform Extraction (SY/T 5118-2005). Ground source rock samples were extracted using chloroform in a Soxhlet extractor and kept in water bath (80℃) for 48 hours. Chloroform bitumen content was then obtained after rotary evaporation and nitrogen gas drying. It was further fractionated into four fractions including saturates, aromatics, non-hydrocarbons
and
asphaltenes
by
the
elution
of
n-hexane,
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and
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chloroform-ethanol. The tests followed the Standard oil and natural gas of the People's Republic of China, Analysis Method for Fractions of Rock Extract and Crude Oil (SY/T 5119-2008). Total hydrocarbon content and Tmax were obtained by Rock-Eval OGE-Ⅱ pyrolysis. The samples were heated to 300℃ for three minutes and thermally evaporated to determine the free hydrocarbon content (S1). The kerogen thermal cracking hydrocarbon content (S2) was measured at the temperature range from 300℃ to 600℃ and the corresponding peak temperature was Tmax. The tests followed the State Standard of the People's Republic of China, Rock Pyrolysis Analysis (GB/T 18602-2012). The paleosalinity recovery methods include the Adams method and Couch method, in which boron concentration in clay minerals is used to determine the salinity of lake. By using the Adams method, Sun et al. (1997) established a classification standard of salinity and built the linear relationship between boron (B) concentration and Cl- concentration in sediments (Figure 3). 2 For the Paleogene and Neogene lakes in the western Qaidam Basin, they were classified as fresh where Clconcentration was less than 0.5 mg/g, brackish where Cl- concentration was 0.5-2 mg/g, saline where Cl- concentration was 2-5 mg/g, high saline where Cl- concentration was 5-8 mg/g, and supersaline where Cl- concentration was more than 8 mg/g.
Figure 3. The Relationship among B, Cl- concentration of formations and paleosalinity of lake reservoirs in the Western Qaidam Basin (revised from ref 2).
4. RESULTS AND DISCUSSION 4.1 Formation salinity temporal and spatial change Based on lithological, geochemical and well logging characteristics, the Paleogene and Neogene sedimentary facies were divided in the western Qaidam Basin. For instance, the E32-N22 sedimentary facies of the cross-section from Well Liang 3 to Well Jian 2 were lacustrine facies including shore-shallow lake and semi-deep lake subfacies (Figure 4). Totally, the Eocene, Miocene and Pliocene sedimentary facies were classified into fluvial, deltaic and lacustrine facies in the western Qaidam Basin (Figure 5, Figure 6, Figure 7). The Cl- concentration test results are listed in the Table
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Figure 4. The Paleogene and Neogene lithofacies crosssection in the western Qaidam Basin.
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1. During the Paleogene and Neogene, the saline lakes in the western Qaidam Basin experienced three evolution stages of origin, development and extinction. These lakes originated from the Paleocene to early Oligocene, when a dry climate48 and strong evaporation in shallow brackish lakes resulted in precipitation of large amounts of carbonates (calcium carbonate). The relatively deeper rift lakes were around the Shizigou, Nanyishan and Xiaoliangshan structures, where Cl- concentration of source rocks was 2-3 mg/g (Figure 5). The development stage of these lakes ranged from the late Oligocene to Miocene, when the climate changed to semiarid48 and fresh water influx increased. The lakes expanded and source rocks were well developed in the depocenters, where Cl- concentration of source rocks ranged from 3-5 mg/g (Figure 6). In the Pliocene, these lakes dried out and the basins were filled with evaporites and coarse clastic rocks. Cl- concentration of source rocks exceeded 7 mg/g in the depocenters of the reducing saline lakes in the northwestern Qaidam Basin (Figure 7). Totally, paleo-water centers gradually migrated eastward, medium and high salinity distribution areas correspondingly migrated eastward, and paleo-water salinity rose from the Paleocene to Pliocene in the western Qaidam Basin.
Figure 5. Distribution of sedimentary facies and Cl- concentration of the lower segment of Xiaganchaigou Formation (E31) source rocks in the western Qaidam Basin.
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Figure 6. Distribution of sedimentary facies and Cl- concentration of Shangganchaigou Formation (N1) source rocks in the western Qaidam Basin.
Figure 7. Distribution of sedimentary facies and Cl- concentration of Shizigou Formation (N23) source rocks in the western Qaidam Basin.
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4.2 Influence of salinity on enrichment of source rocks 4.2.1 Relationship between salinity and organic matter contents The relationships of organic matter content (TOC content and chloroform bitumen content) and Cl- concentration of the Paleogene and Neogene lacustrine source rocks show a clear pattern in the western Qaidam Basin. During the low salinity stage (Cl- concentration less than 1mg/g), both TOC content and chloroform bitumen content have lower values (the minimum TOC content is 0.04%, the maximum is 3.16%, and the mean is 0.56% among 99 shale samples; the minimum chloroform bitumen content is 0.01%, the maximum is 0.13%, and the mean is 0.04% among 63 shale samples) because of the interaction between higher organic productivity and oxidation in fresh and brackish water. There are a baseline (TOC
