Practical application of reservoir geochemistry in petroleum exploration

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Practical application of reservoir geochemistry in petroleum exploration: a case study from a Paleozoic carbonate reservoir in the Tarim Basin (NW China) Meijun Li, Tie-Guan Wang, Zhongyao Xiao, Ronghui Fang, Zhiyong Ni, Weilong Deng, Youjun Tang, Chunming Zhang, and Lu Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03186 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Practical application of reservoir geochemistry in petroleum exploration: a case study from a Paleozoic carbonate reservoir in the Tarim Basin (NW China) Meijun Lia∗, T.-G. Wangb, Zhongyao Xiaoc, Ronghui Fangb, Zhiyong Nib, Weilong Dengb, YoujunTanga, Chunming Zhanga, Lu Yangd a

Key Laboratory of Exploration Technologies for Oil and Gas Resources, Ministry of Education,

College of Resources and Environment, Yangtze University, Wuhan 430100, China b

State Key Laboratory of Petroleum Resources and Prospecting, College of Geosciences, China

University of Petroleum, Beijing 102249, China c

Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina,

Xinjiang Korla 841000, China d

Biogas Institute of Ministry of Agriculture, Sichuan Chengdu 610041, China

Abstract Reservoir geochemistry has a practical application in petroleum exploration. A typical Paleozoic carbonate oilfield was selected from the Tabei Uplift of the Tarim Basin (NW China) to exhibit the method, application and exploration implications of reservoir geochemistry. Oil-oil correlation indicates that all oils analyzed in this study belong to one single oil group. The overall oil migration direction traced by selected organic molecular markers is from the south to the north region of the Halahatang region. The source kitchen for current oil accumulations in the carbonate reservoir is predicted to locate to the south of this oilfield, most likely between the Awati and Manjiaer depressions.

Based on the characteristics of hydrocarbon-bearing

inclusions and the histograms of the homogenization temperatures (Th) and ice-melting temperatures of associated aqueous inclusions, the oil charging temperatures were obtained. The stratigraphic-burial and geothermal histories for

∗

Corresponding author. Tel: +86 108 973 1709, Fax: +86 108 973 1109. Email address: [email protected] (M. Li).

1

ACS Paragon Plus Environment

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representative individual well were reconstructed using 1-D basin modeling. We concluded that the Paleozoic oil reservoir has been charged twice during its oil charging history; firstly from 419 Ma to 410 Ma and secondly from 16 Ma to 8 Ma. The preservation condition for early filling oil accumulations and the mixture of oils charged during the two filling phases have controlled the density and chemical compositions of present oil accumulations. The filling points and preferential pathway indicated by isopleth maps of molecular geochemical indicators are highly indicative of oil reservoirs with high yields. It is concluded that reservoir geochemistry can be utilized, not only to determine oil migration direction and to predict the location of source kitchens, but also favorable charging pathway and potentially prolific prospecting zones. This study suggests that traps in the southern region along the preferred oil charging pathway into the Halahatang Oilfield could be the most favorable targets for further oil exploration in this region. Key words: reservoir geochemistry; oil migration orientation; filling pathway; timing and episodes of oil charging; fluid inclusion; preservation condition; physical and chemical properties

2


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1. Introduction As a typical Paleozoic carbonate oilfield in the Tarim Basin (NW China), the Halahatang Oilfield lies in the south part of the Tabei Uplift (Fig. 1). In recent years, significant oil reserves have been discovered in this region.1-4 The Paleozoic oil reservoir mainly occurs within Ordovician strata, including Lower–Middle Ordovician Yingshan (O1-2y) and Upper Ordovician Lianglitage (O3l) formations. Petroleum with various density, including condensates, light oils and heavy oils to ultra-heavy oils2, 5, 6 have all been discovered in this oilfield. Two oil families have been classified in the Paleozoic reservoir of the cratonic region of the Tarim basin based on various organic geochemical characteristics7-11. Except for a couple of oil samples, the majority of crude oils discovered to date in the Paleozoic reservoir of the Tabei Uplift is considered to source from the Middle-Upper Ordovician carbonates.7,10,12-14 Previous studies indicated that the oils in the Halahatang region resemble those of the Tahe Oilfield in the Tabei Uplift and mainly sourced from Middle-Upper Ordovician source rocks2, 4, 6 At present, reservoir geochemistry has been widely applied in petroleum exploration, reservoir appraisal and development15. The study of the geochemistry of a specific reservoir can reveal the distribution and origin of petroleum, water and minerals in the reservoir15. During exploration stage, reservoir geochemistry can reconstruct the oil charging direction and pathway, determine the preferential filling point and predict source kitchen as well as potential locations for future wells.15 Oil migration direction and charging pathway in the north and north-central parts of the Halahatang Oilfield have been identified using selected molecular indicators in previous studies.3,4,6 Based on observation of hydrocarbon inclusions, measurement of homogenization temperatures of fluid inclusions in reservoir rocks, and the 3


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reconstruction of stratigraphic burial curves and geothermal histories in representative wells, petroleum charging events for the Paleozoic reservoir in this region have been preliminarily determined in previous literature. 3, 6 During past five years, a great number of exploration/development wells with commercial yields have been discovered, especially in the south part of the Halahatang region. Oil families are classified for most of the oil wells discovered to date in this paper. The timing and episodes of oil charging for the Ordovician reservoir in the south part of the Halahatang Oilfield are suited based on the observation of fluid inclusions, homogenization temperatures and ice-melting point temperature measurements, using confocal laser scanning microscopy (CLSM), PVT simulation and 1-D basin modeling techniques. Based on systematic geochemical analyses of more than 100 oil samples, the spatial variations in chemical composition and physical properties between the samples are described. Petroleum filling histories and geochemical backgrounds and how they have influenced spatial variations in physical and chemical properties, and the distribution of wells with high petroleum yields are comprehensively discussed. This study may have significant implication in petroleum exploration in the Halahatang region, by predicting the location of potential source kitchen and favorable exploration targets. It also provides an excellent example for the study of reservoir geochemistry in other basins.

2. Geological settings and samples The Tarim Basin is one of the most prolific hydrocarbon-bearing basins in northwestern China. It lies in the central-southern Xinjiang Uygur Autonomous Region. The Tarim Basin is one of the largest foreland basins in the world, with an area of 56×104 km2 (Fig. 1). The Tabei Uplift is one of the main structural belts of the 4


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Tarim Basin. A number of commercial oil fields with moderate to high reserves have been discovered in this belt. The Tabei Uplift formed during the Sinian to Devonian periods. Then it was followed by interbedded strata of marine and terrestrial sediments from Carboniferous to Permian. During Jurassic to Paleogene, the Tabei Uplift continuously subsided followed by a rapid subsidence from the Neogene to Quaternary. The geological background of the Tarim Basin, especially the Tabei Uplift have been summarized at length in numerous references.8, 16-19 A total of 10 reservoir rock samples from the production interval of Well JY4 in the south part of the Halahatang Oilfield were analyzed to investigate the characteristics of fluid inclusions, the distribution of homogenization and ice-melting temperatures of aqueous inclusions. More than 30 drilled cores and cuttings were collected from Well JY4 and their thermal maturity profiles were established on the basis of measurements of vitrinite reflectance. A total of 105 sample were collated from the Ha6, Xinken (XK), Repu (RP), Jinyue (JY), Qige (QG) and Yueman (YM) blocks of the Halahatang region. Oils from wells T904, TD2, Ma4, Ma401 and TZ162 are thought to be derived from the Cambrian-Lower Ordovician source rock in previous studies6, 8, 10, 20, 21. Five oil samples from these wells were also collected and geochemically analyzed for comparison.

3. Methods 3.1. Gas chromatography–mass spectrometry (GC–MS) All oils were geochemically analyzed as following experimental procedure. The oil samples were deasphaltened with 50ml of n-hexane and then fractionated into saturated and aromatic hydrocarbon fractions by liquid chromatography using silica gel/alumina columns using 30 ml n-hexane with dichloromethane/n-hexane (2:1 v/v) 5


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and 20 ml of dichloromethane as eluents, respectively. Gas chromatography–mass spectrometry analyses of the saturated and aromatic fractions were conducted on an Agilent 5975i mass chromatography system equipped with an HP–5MS (5% phenylmethylpolysiloxane) fused silica capillary column (60m × 0.25 mm i.d., with a 0.25 µm film thickness). The gas chromatography (GC) temperature program was as follows: the initial oven temperature was set at 50 °C and 80 °C for 1 min for the saturated and aromatic fractions, respectively. For the saturated fraction, the temperature increased to 120 °C at a rate of 20 °C /min, subsequently to 310 °C at 3 °C /min, and finally held isothermal for 25 min. For the aromatic fractions, it ramped to 310 °C at 3 °C /min, and then kept isothermal for 16 min. The carrier gas was helium and the injector temperature set at 300 °C. The full scan model with a scanning range of 50–600 Dalton was used in the mass spectrometer, which was run by electron impact (EI) at 70 eV.

3.2. Microthermometry of fluid inclusions The discovered oil accumulations in the Halahatang region mainly occurred in the Lower-Middle Ordovician Yingshan Formation (O1-2y). A total of 10 reservoir rock samples were collected from drilled cores of Well JY4. Microthermometric measurements were performed in a fluid-inclusion platform at the State Key Laboratory of Petroleum Resources and Prospecting (Beijing, China). A Linkam Model THMSG 600 heating-freezing stage attached to a Leica Model DMRXP optical microscope was applied for inclusion observation, homogenization and ice-melting temperature measurement. Synthetic fluid inclusions (Fluid Inc.) with temperature of –56.6 °C, –10.7 °C and 0.0 °C were used for stage calibration. The measurement range was between –196 and 600 °C, frozen and the heating errors were 6


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±0.1 °C and ±2 °C, respectively. The heating rate was initially set at 15 °C /min, and then gradually reduced to 5 °C /min and 2 °C /min. It finally decreased to 1 °C /min when the fluid inclusion was close to phase change point.

3.3. Vitrinite reflectance measurement Vitrinite reflectance values (%Ro) were measured on polished rock blocks using a Leitz MPV-3 microscopic photometer at the State Key Laboratory of Coal Resources and Safe Mining (Beijing, China) following standard procedure.22 The reflectance of vitrinite-like (%VLMRo) macerals was measured for Paleozoic rocks and converted to equivalent %Ro through equations established in the Tarim Basin by previous study. 23

4. Results and discussion 4.1. Oil family classification The source and origin of the Ordovician petroleum in the cratonic region of the Tarim Basin has long been controversial, because oils were highly thermally mature and deep wells which penetrate potential source rock intervals are rare.3,

24

The

Cambrian to Lower Ordovician and Middle-Upper Ordovician strata are potential source beds for petroleum accumulations in the Ordovician reservoir.

25-28

Various

molecular markers and related geochemical indicators have been applied in oil population classification and oil-source correlation for the Ordovician reservoir in the cratonic region of the Tarim Basin. In this study, a total of 15 geochemical parameters that have been successfully used in previous studies in the Tabei Uplift were employed in oil family classification in the Halahatang Oilfield. The parameters, and a summary of their geochemical implications, are listed in Table 1. Their geochemical significance and their application in oil-oil and oil-source correlation in the Uplift of 7


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the Tarim Basin are described in detail in the relative publications. Hierarchical cluster analysis (HCA) is a useful tool for the classification of crude oils derived from different source rocks.29-31 By using the molecular geochemical indicators referred to above in conjunction with the stable carbon compositions (Table 1) of samples, oil families can be determined using the HCA method. A dendrogram (Fig. 2) shows that two distinct oil families can be identified in the Ordovician carbonate petroleum reservoir in the cratonic region of the Tarim Basin. All the analyzed oils in the Halahatang Oilfield belong to the same oil population (Fig. 2) and are likely to originate from same the source bed/source kitchen. Therefore, the variations in selected molecular markers can be used to trace oil migration orientation and filling pathway.

4.2. Oil charging orientation and filling pathway Concentration of some polar organic compounds and the relative abundance of their isomers in oils change due to their differences in absorption of these compounds onto immobile mineral surfaces in carrier beds during oil migration.32 Various molecular indicators relative to pyrrolic nitrogen compounds,33-35 dibenzothiophene and benzo[b]naphthothiophenes,

4,

36-38

and even some polycyclic aromatic

hydrocarbons 39 have been proposed and applied as indicators in tracing oil migration directions and filling pathway. Previous study shows that both differences in molecular thermodynamic stability and absorption onto immobile mineral surface can result

in

migration

fractionation

of

4-methyldibenzothiophene

and

1-methyldibenzothiophene isomers. 36-38 Using some indictors, such as 4-/1-MDBT (4-/1-methyldibenzothiophene), [2,1]BNT/([2,1]BNT+[1,2]BNT) (BNT: the ratio of benzo[b]naphtho[2,1-d]thiophene 8


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to benzo[b]naphtho[1,2-d]thiophene) ratios, C2S/C2E (the ratio of shield C2-carbazole to exposed C2-carbazole),3, 4, 6, 38 oil migration directions and filling pathway for the Ordovician reservoir in part of the Halahatang Oilfield were traced. The results indicate that oil accumulations in the Halahatang Oilfield primarily migrated from the south to the north region. More than 100 oil samples were geochemically analyzed and several migration indicators were obtained. Fig. 3 shows the isopleth map of 4-/1-MDBT (the relative abundance of 4-methyldibenzothiophene to 1-methyldibenzothiophene) ratio, in which the direction of decrease of 4-/1-MDBT ratios indicates the oil migration direction, and the projecting loci of 4-/1-MDBT isopleths indicate the probable preferential oil charging pathway. It can be seen that there is a predominant oil filling point around Well YM5 and a minor filling point around Well YM3. A principal oil stringer migrates from filling point wells YM5 to RP7008, Ha13-5 and Ha11-3. Four sub-streams can be divided along the main Northward migrating oil stream, two of which migrate northeastwards towards the Ha6 blocks and the other two streams charge northwestward the Xinken and Repu blocks. The overall migration orientation and filling pathway identified in this study are generally consistent with previous results in other studies4, 6 and the distribution of fracture-cavity zones.40

4.3. Episodes and timing of oil charging A decade core samples were collected from the production intervals of the O1-2y Formation in drilled well (JY4). The characteristics of fluid inclusions, including the compositional types, vapor-liquid ratios, spatial clustering and the fluorescent color of hydrocarbon-bearing inclusions were comprehensively analyzed. Homogenization temperatures of both hydrocarbon-bearing inclusions and associated aqueous 9


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inclusions were measured. The sparry calcite is the host mineral for homogenization temperature measurement. Both hydrocarbon-bearing and aqueous inclusions occur widely in sparry calcite formations (Fig. 4). Three types of fluid-inclusions were identified, based primarily on their UV-epifluorescence nature and their phase at room temperature. Hydrocarbon-bearing inclusions exhibit principally ‘yellow’ and secondarily ‘bluish– white’ colors under UV-epifluorescence, which indicates mature and highly-mature oil, respectively. This may indicate two stages of oil charging. Aqueous inclusions, paragenetic with the above-mentioned two types of hydrocarbon-bearing inclusions, were also observed and their corresponding homogenization temperatures were measured as well. The homogenization temperatures (Th) and ice-melting temperatures apparently differ between these two phases of fluid inclusion formation (Figs. 5). The histogram plotted on 74 measured homogenization temperature data points of aqueous inclusions exhibits a bimodal distribution pattern (Fig. 5a). The temperature ranges from 80 °C to 90 °C for the first major frequency and 100 °C to 110 °C for the second one (Fig. 5a). Therefore, there are apparently two different trapping temperatures of aqueous inclusions, suggesting two oil filling events occurred in the Ordovician reservoir. Moreover, the ice-melting temperatures range from‒20 °C to ‒12 °C and‒10 °C to ‒6 °C for the first and second phase, respectively (Fig. 5b), which clearly distinguish the diagenetic palaeo-fluid properties of the two phases of fluid inclusions. The subsurface water of first phase should be more saline than that of the second one. Therefore, the fluorescence colors of hydrocarbon-bearing inclusions, the measured homogenization temperatures and ice-melting temperatures of associated aqueous inclusions all indicate two distinct petroleum charging and accumulating events. 10


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It

is

generally

known

that

homogenization

temperatures

are

the

lowest-entrapment temperature of the fluid inclusions and so do the pressures.41 Because the volume of the fluid inclusions is constant, the temperature and/or pressure of fluid inclusions must therefore vary along with the isochors thereof. This study utilized confocal laser scanning microscopy (CLSM) and pseudo-3D images to obtain the vapor/liquid ratios of seven individual petroleum-fluid inclusions. Then we calculated the isochors of the petroleum fluids (Fig. 6) using commercially available PVT simulation software PVTSIM 20 by combining these data with the homogenization temperatures and phase envelopes of the petroleum fluids.

42

Particularly, each pair of hydrocarbon-bearing inclusions and aqueous inclusions co-occurs with each other in the same sparry calcite deposits (as observed under microscopy). Thus, the intersection of these two isochors can be obtained in the P-T coordinate system, indicating the trapping temperature (Ttrap) and pressure (Ptrap) of an identifiable pair of inclusions.43-45 The results show that the entrapment temperatures for these two phases of fluid inclusion in Well JY4 ranges from 89 °C to 128 °C and 102 °C to 168 °C, respectively. The calibrated temperatures of aqueous inclusions are ≈20 °C higher than the measured homogenization temperature for the first phase and ≈30 °C higher for the second phase. Therefore, the trapping temperatures of fluid inclusions in the Ordovician reservoirs of Well JY4 are in the ranges of 90 °C to 100 °C, and 130 °C to 150 °C, respectively. By means of BasinMod 1-D numerical modeling, this study reconstructed the stratigraphic-burial and thermal histories for Well JY4 (Fig. 7). Stratigraphic data were collected from the completion reports of the Tarim Oilfield Company. The geological data including formation events, chronostratigraphy, temperature, erosional event, palaeobathymetry and sea-level variations are referred to our previous study. 46 11


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Default parameters (e.g. porosity, matrix density, etc.) in the BasinMod 1-D software were used for initial values. Especially, the measured vitrinite reflectance (%Ro) profile was established in this study for calibration of the geothermal history (Fig. 8). The values of geological parameters were reasonably adjusted until the calculated and measured vitrinite reflectance profiles are overall consistent with each other.46 Figure 8 shows that the model of stratigraphic burial and thermal histories for Well JY4 in this study is one of the most reliable ones. Therefore, the approximate oil filling and entrapment temperatures determined through fluid inclusion can be directly converted to a specific geological age. Figure 7 illustrated the stratigraphic burial and thermal curves and corresponding geological ages converted from the homogenization temperatures. The results show that the timing of oil entrapment of Upper Ordovician Yingshan Formation in Well JY4 is from 419 to 410 Ma and from 16 to 8 Ma, respectively, which are generally in agreement with those of wells Ha9 and RP7 in the Halahatang region

3, 6

and wells

S47, T401, S67, S76 in the Tahe Oilfield in the Tarim Basin 9. The oil charging episodes are also consistent with the oil generation history of the Paleozoic source kitchen for the Tabei Uplift.46,47 The deposition of Silurian with thickness more than 3000 m resulted in the first oil generation phase of Cambrian-Ordovician source rocks.46 The hydrocarbon generation process ceased because of the uplift and erosion of Silurian strata. The burial depth of Paleozoic source rocks during Devonian to Paleogene was not greater than that deposited in late Silurian due to the multiple uplift, erosion and deposition events. The rapid deposition and thick Neogene strata resulted in the second phase of hydrocarbon generation.

4.4. Factors controlling the physical and chemical properties of oils 12


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For oil accumulations from the same source kitchen/bed, physical and chemical properties are mainly controlled by preservation conditions and oil charging phases. This paper and previous studies all indicate that the discovered oil in Paleozoic carbonate reservoirs in the Tabei Uplift have undergone two petroleum charging events. The late oil charging phase occurred in the Late Miocene and was dominated by light oils with higher maturation levels, which have ideal preservation conditions. Preservation conditions for oils from the early charging phase play a vital role in variations in the physical and chemical properties of oils. Petroleum discovered in the Halahatang Oilfield varies considerably in its physical properties. For example, light oils with density lower than 0.80 g/cm3 and ultra-heavy oils with density higher than 1.00 g/cm3 have all been discovered in this oilfield.

2, 5, 6

Heavy oils (>0.9200 g/cm3) mainly occur in the north part of the

Halahatang Oilfield (Fig. 9), particularly in wells XK1, Ha6 Block, and in Well AD4 Block. Densities gradually decrease from north to south (Fig. 9). The majority of oil accumulations occur in the Middle-Lower Ordovician Yingshan (O1-2y) Formation. Therefore, the thickness of Silurian and Upper Ordovician sediments is the crucial factor controlling the physical properties of the oils. A contour map of Silurian and Upper Ordovician sediments based on strata thickness revealed by boreholes was presented in the study of Ni et al.45 The thickness of Silurian plus Upper Ordovician (S+O3) strata increases from north to south. It is approximately only 200 m thick in the north part of the Halahatang Depression (Fig. 9). It gradually increases to more than 1700 m thick in the south part. Figure. 9 shows that heavy oil, with density higher than 0.92 g/cm3 mainly occurs in the Ha6 Block and the north part of the XK Block, where the thickness of Silurian and Upper Ordovician strata is thinner than 600m. Therefore, the 13


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Silurian and Upper Ordovician strata serve as regional seal rocks for oils accumulated during the early entrapment event (Late Silurian period). The oils charged during the later episode are dominated by light oils with high maturity. For the thick strata of Silurian and the shallower Formation, oils accumulated during this stage were almost unaffected by secondary alteration. Therefore, the density of oil accumulations in the Halahatang Oilfield is dependent on the thickness of Silurian and Upper Ordovician strata and the proportion of oils from the two charging phases. For example, some oils in the Ha6 block with S+O3 thickness less than 600m are normal heavy oils and even light oils (Fig. 9) may result from a major contribution by oils from the late charging phase. Ordovician heavy oils principally occur in the northeast part of the Ha6 Block. The gas chromatograms of these oils are characterized by the distribution of an intact n-alkane series and an apparent “hump” (UCM, i.e., unresolved complex mixture) of the base line (Fig.10). The compound of 25-norhopanes are also detected on m/z 177 mass chromatograms of the same oil samples. The occurrence of 25-norhopane and apparent UCM indicate that the oils were severely biodegraded. While the intact n-alkanes series suggest that the oils were not biodegraded. Therefore, their coexistence in same oil sample may result from the mixing of oils from two separate oil filling processes occurring in the Ordovician oil reservoir of the Halahatang, i.e. late filled oil was mixed with severely biodegraded oils from an earlier filling event.48, 49

However, no 25-norhopane series were detected in oils in the south part of

Halahatang Depression (Fig.11), even though two episodes of oil filling also occurred there. This means that early filled oils in the south part of the depression did not experience significant secondary alteration due to the thick seal rocks of Silurian and Upper Ordovician strata. 14


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4.5. Oil charging process predicting the preferential exploration region The reservoir geochemical study aforesaid indicates that all the Ordovician carbonate oils in the Halahatang Oilfield belong to same oil population. They were derived from the same source kitchen/bed and experienced similar filling history. The tracing results in section 4.2 showed that the oils primarily charged from south to north. The main oil filling point identified by samples from oil wells in this study is located around Well YM5. Therefore, it can reasonably predict that the source kitchen/bed for the Ordovician reservoir of the Halahatang Oilfield is most probably located to the south, most likely at the Manxi (Shuntuoguole) Low-Uplift, which is situated between the Manjiaer and Awati Depressions (Fig. 1). This location of the source kitchen/bed predicted in this study roughly coincides with those in previous studies.3, 4, 9 Well blocks with petroleum yields higher than 50t/d and 80t/d are shown in Fig. 3 Preferential charging pathway indicated by molecular markers is generally consistent with well blocks with high yields (Fig. 3), which indicates that the preferential pathway predicted in this study are of practical use in suggesting areas for future exploration. As an example, Well Qi2C, which is not located in the preferential filling pathway (Fig. 3), has much lower oil production than those in the identified filling pathway. Therefore, we suggest that the Well Qi2C block is not a worthwhile prospecting target and that no more exploration wells should be drilled there in future unless more favorable geochemical data become available. Recently, commercial oil flows in have been drilled in considerable number of wells in the south part of the Halahatang.

in which he petroleum discovered in this region is

dominated by light oils and condensates. 15


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The Halahatang petroleum reservoir is a typical ancient carbonate reservoir with multiple charging events. The method to reservoir geochemistry in this study may have reference value for other basins of this sort. The preservation condition, especially, the thickness of caprocks for oils early charged and mixing ratio of oils late filled are critical to the physical and chemical properties of current oil accumulation. 5. Conclusions A comprehensive reservoir geochemical study of the Ordovician carbonate oils in the Halahatang region of the Tarim Basin (northwest China) presents a typical case study of reservoir geochemistry in hydrocarbon exploration. All oils from the Ordovician carbonate reservoir in the Halahatang region show a high level of chemical similarity and most probably belong to one oil family, which indicates that they could share a common petroleum charging history and be derived from a single common source bed/kitchen. The characteristics of hydrocarbon-bearing inclusions exhibit two distinct phases. A histogram plotted on measured homogenization temperatures (Th) of associated aqueous inclusions in the Ordovician reservoir rocks in the Halahatang region shows a bimodal distribution pattern, which indicates that two petroleum charging events have occurred for Ordovician oil reservoir during its oil charging history. Combined the measured Th (ºC) with burial and geothermal histories reconstructed by 1-D numerical modeling, the Th (ºC) was converted to relevant geological ages of 419 Ma to 410 Ma and 16 Ma to 8 Ma. The oil migration orientation of the Ordovician carbonate reservoir was determined using an isopleth map of its organic molecular parameters. The tracing results show that oils mainly migrated and charged from south to north with several 16


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oil filling points and preferential filling pathways. Therefore, this study predicted that the potential source kitchen is located on the south part of the Halahatang Oilfield, most probably at the Manxi Low-Uplift between the Manjiaer and Awati depressions. As the caprocks for the oil accumulated during the early charging phase, the thickness of Silurian and Upper Ordovician strata in the Halahatang region controlled the preservation condition. The oils underwent a certain of degree of biodegradation where the thickness of the Silurian and Upper Ordovician rocks is less than ≈600 m. The proportion of oils accumulated during the late charging pulse may also have affected the physical and chemical properties of present reservoired oils. Acknowledgement This work was funded by the National Science and Technology Major Project (No. 2016ZX05004-005) and the State Key Laboratory of Petroleum Resources and Prospecting (PRP/indep-03-1615). The authors would like to express their appreciation to Shengbao Shi, Lei Zhu, Daowei Wang for the assistance in the GC– MS analysis, and Chengyu Yang (the State Key Laboratory of Petroleum Resources and Prospecting) in the microthermometry of fluid inclusions analysis. We would like to thank the Tarim Oilfield Company of PetroChina for providing samples and data, and for permission to publish this work. Dr. Ryuzo Tanaka and two anonymous reviewers are thanked for their constructive suggestions and comments.

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Captions of figures and tables Fig 1. (a) Schematic map showing the structural elements of the Tarim Basin, NW China, with location of Halahatang Oilfield, and (b) the distribution of oil wells sampled in this study (after Ref. (4)). Fig. 2. Classification diagram of hierarchical cluster analysis (HCA) based on the molecular geochemical parameters of Halahatang oils. Fig.3. Isopleth map of 4-/1-MDBT ratio indicating the migration direction, preferential pathway and distribution of production zones with high oil yields in the Halahatang Oilfield. Fig. 4. Characteristics of typical fluid inclusions in the Ordovician carbonate reservoir of Well JY4 in the Tarim Basin. Fig. 5. Histogram of homogenization temperatures (Th) and ice-melting temperatures (Tm, ice) of fluid inclusions in the Ordovician carbonate oil reservoir in Well JY4. Fig.6. Representative isochors and phase envelopes for two stages of petroleum fluid inclusions, and isochors for coexisting aqueous-fluid inclusions. Fig. 7. Reconstructed stratigraphic burial and geothermal histories, and oil filling and entrapment timings for the Ordovician reservoir in Well JY4. Fig. 8. Comparison of measured vitrinite reflectance VRo and calculated equivalent vitrinite reflectance VRc for Well JY4. Fig. 9. Isopleth maps of the density of carbonate oils and the thickness of caprocks (Silurian and Upper Ordovician) in the Halahatang region. Fig 10. Distribution of “UCM” (unresolved complex mixture) for base lines of gas chromatograms of oils from the Halahatang region. Fig 11. The distribution of relative concentrations of C29 25-norhopane (the ratio of C29 25-norhopane to C30 hopane in m/z 191 mass chromatograms of saturated fraction) 23


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in Ordovician carbonate oils in the Halahatang region. Table 1. Molecular geochemical indicators and their application used for oil classification in this study. Table 1 No.

Geochemical parameter

Application in oil family classification in the Tabei Uplift

1

Pr/Ph

The ratio of pristane to phytane; oils from Middle-Upper 2 Ordovician source rocks having relatively lower values (0.6~1.06).

2

C28-Sterane%

The percentage of C28 regular steranes among C27, C28 and C29 2, 4, 6, 9 homologues. this value lower than 25% in oils sourced from Middle-Upper Ordovician.

3

C28/C29 regular sterane The relative abundance of C28 to C29 regular sterane; oils from 2, 10, 50 Middle-Upper Ordovician source rocks having lower ratios than those from Lower Ordovician-Cambrian source bed.

4

C21/C23 TT

The relative abundance of C21 to C23 tricyclic terpanes; oils from 2, 51 Middle-Upper Ordovician having relatively lower ratios than those from Low-Ordovician and Cambrian source rocks.

5

C29H/C30H

The relative abundance C29 hopane to C30 hopane; relatively higher 2, 52, 53 in carbonate and evaporate sourced oils (0.7 or greater) and lower for shale sourced oils (0.4–0.75).

6

DBTs%

The percentage of dibenzothiophenes among dibenzothiophenes, 2-4, 9, 54 dibenzofurans and fluorenes; oils from strongly reducing depositional environment and a marine carbonate source rocks having DBTs% higher than 60%.

7

ADBT/ADBF

Alkyldibenzothiophenes to alkyldibenzofurans ratio; oils from 26, 55 marine carbonate source rocks having higher ADBT/ADBF ratios (>3.0)

8, 9

C27/C28 20R TAS and The relative abundance of C27 to C28 20R triaromatic steroid and C26 7, 10, 12 C26/C28 20S TAS to C28 20S triaromatic steroid, respectively; these ratios generally lower in oils from Middle-Upper Ordovician source rocks than those form Lower Ordovician-Cambrian source rocks.

10

TDSI

11, 12

BaA/(BaA+Chy) MBaA/2-MChy

13, 14

Fla/(Fla+Py) and These two ratios are the relative abundance of fluoranthene to 21 MFla/(MFla + MPy) pyrene, methylfluoranthenes to methylpyrenes, respectively; empirical indicators to distinguish oils from Middle-Upper Ordovician and Cambrian-Lower Ordovician source rocks.

15

δ C (‰)

13

References

Triaromatic dinosteroids index: triaromatic steroids/(triaromatic 7, 10 dinosteroids + 3-methyl-24-ethyl triaromatic steroids); this ratio very low in the oils from Middle-Upper Ordovician source rocks. and The relative abundance of Fluoranthene to pyrene, 14 methylfluoranthenes to methylpyrenes, respectively; empirical indicators to distinguish oils from Middle-Upper Ordovician and Cambrian-Lower Ordovician source rocks.

13

The δ C (‰) values of whole oils and their fraction groups 2, 51 typically lower than ‒30‰ for oils derived from Middle-Upper Ordovician source rocks. oils from Lower Ordovician-Cambrian 13 source rocks more C enriched.

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Fig 1. (a) Schematic map showing the structural elements of the Tarim Basin, NW China, with location of Halahatang Oilfield, and (b) the distribution of oil wells sampled in this study (after Ref. (4)). 62x31mm (300 x 300 DPI)


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Fig. 2. Classification diagram of hierarchical cluster analysis (HCA) based on the molecular geochemical parameters of Halahatang oils. 207x533mm (300 x 300 DPI)


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Fig.3. Isopleth map of 4-/1-MDBT ratio indicating the migration direction, preferential pathways and distribution of production zones with high oil yields in the Halahatang Oilfield. 163x195mm (299 x 299 DPI)


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Fig. 4. Characteristics of typical fluid inclusions in the Ordovician carbonate reservoir of Well JY4 in the Tarim Basin. 130x196mm (300 x 300 DPI)


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Fig. 5. Histogram of homogenization temperatures (Th) and ice-melting temperatures (Tm, ice) of fluid inclusions in the Ordovician carbonate oil reservoir in Well JY4. 154x80mm (300 x 300 DPI)


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Fig.6. Representative isochors and phase envelopes for two stages of petroleum fluid inclusions, and isochors for coexisting aqueous-fluid inclusions. 140x111mm (300 x 300 DPI)


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Fig. 7. Reconstructed stratigraphic burial and geothermal histories, and oil filling and entrapment timings for the Ordovician reservoir in Well JY4. 133x104mm (300 x 300 DPI)


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Fig. 8. Comparison of measured vitrinite reflectance VRo and calculated equivalent vitrinite reflectance VRc for Well JY4. 102x135mm (300 x 300 DPI)


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Fig. 9. Isopleth maps of the density of carbonate oils and the thickness of caprocks (Silurian and Upper Ordovician) in the Halahatang region. 167x203mm (299 x 299 DPI)


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Fig 10. Distribution of “UCM” (unresolved complex mixture) for base lines of gas chromatograms of oils from the Halahatang region. 265x310mm (150 x 150 DPI)


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Fig 11. The distribution of relative concentrations of C29 25-norhopane (the ratio of C29 25-norhopane to C30 hopane in m/z 191 mass chromatograms of saturated 189x219mm (299 x 299 DPI)


Practical application of reservoir geochemistry in petroleum exploration

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