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Organic properties, formation environment and their controlling factors of two effective marine source rocks in the Tarim Basin
... thermal history of source rocks controls the timing of hydrocarbon generation and expulsion (Carminati et al., 2010;Hudson and Hanson, 2010). The hydrocarbon source rocks in the Tarim Basin are mainly found in the Cambrian and Ordovician strata, and they are deeply buried and mostly thermally overmature (Zhao et al., , 2008S. C. Zhang et al., 2000S. C. Zhang et al., , 2001B. M. Zhang et al., 2005;Mu, 2009). The early Paleozoic thermal history was an important factor in the evolution of the lower Paleozoic source rocks because Paleozoic strata may achieve thicknesses of up to 7000 m (22,966 ft). Previous studies have reported opposing Paleozoic thermal histories, among which most researchers have agreed th ...
... The depocenter moved to the northern part of the basin and the Southwest and Southern depressions during the Mesozoic. Several depocenters developed in front of the Tianshan and Kunlun mountains during the Cenozoic (Figure 3 Zhang et al., , 2001Zhao et al., 2008;Mu, 2009). They are currently buried to depths of more than 5000 m (16,404 ft) in most areas of the basin, and as a result, these lower Paleozoic source rocks are at a high-mature or overmature stage . ...
... They are currently buried to depths of more than 5000 m (16,404 ft) in most areas of the basin, and as a result, these lower Paleozoic source rocks are at a high-mature or overmature stage . The black and dark-gray Ordovician mudstones and mudrich limestones are the major source rocks for the Ordovician reservoirs (S. C. Zhang et al., , 2001B. M. Zhang et al., 2005;Mu, 2009). ...
The Tarim Basin is one of the richest basins in oil and gas resources in China. The Cambrian and Middle Upper Ordovician strata are the most important source rocks. Previous early Paleozoic thermal histories have led to varied hypotheses on the evolution of the lower Paleozoic source rocks, causing a significant impact on petroleum exploration in the basin. A new Paleozoic thermal history of the Tarim Basin was reconstructed in this article using the integrated thermal indicators of apatite and zircon (uranium-thorium)/helium ages, apatite fission tracks, and equivalent vitrinite reflectance data. The modeled results indicate that different parts of the basin experienced widely differing early Paleozoic thermal gradient evolution. The eastern and central regions of the basin experienced a decreasing thermal gradient evolution from 37 to 39 degrees C/km during the Cambrian and Ordovician to 35 to 36 degrees C/km in the Silurian, whereas the northwestern region of the basin had an increasing early Paleozoic thermal gradient evolution from 28 to 32 degrees C/km in the Cambrian to 30 to 34 degrees C/km in the Ordovician and Silurian. The Lower Cambrian thermal gradient resulted from the higher thermal conductivity of the 800- to 1000-m (2625- to 3280-ft) thickness of gypsum and salt in the Cambrian strata. The basin experienced an intracratonic phase during the late Paleozoic and a foreland basin phase during the Mesozoic and Cenozoic, with the thermal gradient decreasing to the present-day value of 20 to 25 degrees C/km. The sensitivity of thermal modeling by the best-fit method is less than +/- 5% in our study, and the differences of the early Paleozoic thermal gradient evolution in different regions of the basin may result in differential maturation of lower Paleozoic source rocks. The maturity histories of the source rocks, modeled based on the new thermal histories, indicate that the lower Paleozoic source rocks in most areas of the basin matured rapidly and reached the late mature to dry-gas stage during the Paleozoic but experienced slower maturation during the Mesozoic and Cenozoic. These new data on the Paleozoic thermal history and lower Paleozoic source rock maturity histories of the Tarim Basin provide new insights to guide oil and gas exploration of the basin.
... Many studies have demonstrated that source rocks exhibit strong heterogeneity in both marine and continental facies at both macro and micro scales [16,17]. Based on the geochemical analysis of eight coring wells, it was discovered that the Chang 7 member shale shows strong vertical heterogeneity. ...
During the Mesozoic, the T-J1 oil system of the Ordos Basin, whilst the degree of oil enrichment, main production layer, and source rock distribution exhibit strong regional differences, no systematic study has been conducted to investigate these differences. At this time, the total organic carbon abundance and vertical distribution of the eight long core wells in different areas of the basin within the Chang 7 member source layers were calculated by means of the ΔLogR method. According to the industrial oil well and the low production well, the favorable oil distribution areas of the Chang 8, Chang 7, and Chang 6 reservoirs are demarcated. The current study confirmed five distribution styles and strong regional differences in the longitudinal direction of source rocks. To be more specific, the Jiyuan area in the northwestern part of the lake basin is dominated by the bottom rich type and the full section rich type. The northeastern Shaanxi region is mainly dominated by the middle rich type and the top rich type. Meanwhile, the central area of the basin is mainly the interlayered type, and the southwestern Longdong region is mainly the bottom rich type. The comprehensive analysis of source rock type and oil favorable zone revealed that source rock type has a controlling effect on the crude oil distribution. The bottom rich type and full section rich type dominate the Jiyuan area and multiple layer oil production. In northern Shaanxi, the top rich type and middle rich type accumulate on the upper portion. Also, the Chang 6 reservoir was the main production layer. The bottom rich type of the Longdong area accumulates under the source, while the Chang 8 reservoir is the main production layer. The central parts of the lake basin are dominated by the interlayered type with multiple layers of production oil. The close relationship between the distribution pattern of source rocks and oil accumulation indicates an improvement on the distribution law of the continental lake with significance practical implication on the optimization of the field of near-source-in-source oil and gas exploration. Keywords: Ordos basin, Yanchang formation, Patterns of source rocks, Exploration direction
... The Lower Paleozoic in the Tarim Basin develops two sets of marine source rock: Cambrian and Upper-Middle Ordovician [12,18,19] , which are distributed widely in the basin and covers an area of more than 20×10 4 km 2 . The multi-stage hydrocarbon generation and expulsion results in rich hydrocarbon resources, and the petroleum resource evaluation shows the hydrocarbon generation volume is up to 10×10 12 t. ...
A comprehensive analysis of hydrocarbon accumulation conditions and typical reservoirs reveals that a large carbonate petroleum province is formed around the Manjiaer depression in the Tarim Basin and it consists of Ordovician non-structural petroleum reservoirs. Multi-type litho-stratigraphic reservoirs are stacked vertically and connected laterally along the paleo-uplift slopes, with a large area. Reservoirs are intensively heterogeneous, extensively developed and distributed in layers along the top of different-scale sequence boundaries. Hydrocarbon phase and physical properties are diverse and the distribution pattern is “lower gas, upper oil”. The oil/gas/water output alters and declines quickly, and 20%-30% high-efficiency wells provide more than 70% of the production. The stable paleo-uplift, widely developed heterogeneous reservoir, multi-stages of hydrocarbon accumulation are the basic forming and distribution conditions of the large carbonate petroleum province. The paleo-uplift near the hydrocarbon kitchen is developed stably and controls the reservoir distribution, migration and accumulation. The special characteristics of carbonate reservoirs result in the wide and discontinuous distribution of irregular fracture-cavity hydrocarbon pools. The enrichment of hydrocarbon is controlled by reservoirs, which are the main controlling factor of the formation and distribution of the large petroleum province. Multi-stages of accumulation and adjustment of hydrocarbon are universal and result in the large area and diversity of the oil and gas.
... The developing environment of hydrocarbon is mainly basins, slopes and depressions within a platform. The marine source rocks in the Tarim Basin are located in the Cambrian and Ordovician [21,22] , while those in the Sichuan Basin are developed in the Early Cambrian, Early Silurian and Late Permian [23] . Well developed source rocks are founded in the Devonian in Guizhou and Guangxi. ...
The Paleozoic strata in the Tarim Basin, Sichuan Basin and Ordos Basin are the major targets for marine petroleum exploration,
with developed high quality hydrocarbon, mainly argillite. The deep burial of these source rocks suggests that they mainly
develop gas instead of oil. But different maturities of organic matter may lead to different hydrocarbon facies. Through thermochemical
sulfate reduction (TSR), the hydrocarbon in the carbonate rocks may undergo a process of pyrolytic cracking and be catalyzed
into gases. The marine reservoirs mainly consist of carbonate and clastic rocks, and the former is controlled by sedimentary
facies, dolomitization, solution, TSR and cracking. The multiphase tectonic cycling develops multiple source-reservoir-cap
combinations and diversified types of traps and reservoirs, featuring multiphase reservoir formation, mainly late-phase formation
or consolidation. Palaeo-uplifts play a controlling role in hydrocarbon accumulation. Differences in major source rocks in
the three basins lead to different locations of oil-gas accumulation layers, different types and patterns of reservoirs and
different features of reservoir formation.
The Tarim Basin is located in northwestern China and is the biggest basin in China with huge oil and gas resources. Especially the Lower to Middle Cambrian and Middle to Upper Ordovician possess the major marine source rocks in the Tarim Basin and have large shale gas resource potential. The Cambrian–Ordovician shales were mainly deposited in basin–slope facies with thicknesses between 30–180 m. For shales buried shallower than 4500 m, there is high organic matter abundance with TOC (total organic carbon) mainly between 1.0% and 6.0%, favorable organic matter of Type I and Type II, and high thermal maturity with RoE as 1.3%–2.75%. The mineral composition of these Cambrian–Ordovician shale samples is mainly quartz and carbonate minerals while the clay minerals content is mostly lower than 30%, because these samples include siliceous and calcareous shale and marlstone. The Cambrian and Ordovician shales are compacted with mean porosity of 4% and 3%, permeability of 0.0003×10–3–0.09×10–3 μm2 and 0.0002×10–3–0.11×10–3 μm2, and density of 2.30 g/m3 and 2.55 g/m3, respectively. The pores in the shale samples show good connectivity and are mainly mesopore in size. Different genetic types of pores can be observed such as intercrystal, intergranular, dissolved, organic matter and shrinkage joint. The reservoir bed properties are controlled by mineral composition and diagenesis. The maximum adsorption amount to methane of these shales is 1.15–7.36 cm3/g, with main affecting factors being organic matter abundance, porosity and thermal maturity. The accumulation characteristics of natural gas within these shales are jointly controlled by sedimentation, diagenesis, hydrocarbon generation conditions, reservoir bed properties and the occurrence process of natural gas. The natural gas underwent short‐distance migration and accumulation, in‐place accumulation in the early stage, and adjustment and modification in the later stage. Finally, the Yulin (well Y1) and Tazhong (well T1) areas are identified as the targets for shale gas exploration in the Tarim Basin.
According to the chemical, carbon isotopic, and hydrogen isotopic compositions and ³He/⁴He ratios of 127 natural gas samples, the natural gas in the Tarim Basin is mainly composed of gaseous alkanes dominated by methane. The contents of the gaseous alkanes decrease with the increase of their carbon numbers. The δ¹³C values of CH4, C2H6 and C3H8 in the platform area of the Tarim Basin vary in the range of −54.4‰∼-24.4‰ (an average value of −39.2‰), −43.1‰∼-26.7‰ (an average value of −36.1‰) and −37.9‰∼-21.6‰ (an average value of −31.8‰), respectively. The overall carbon isotopic composition of the alkanes follows the trend of δ¹³C1< δ¹³C2< δ¹³C3< δ¹³C4 and in some cases varies with δ¹³C1>δ¹³C2 or δ¹³C3> δ¹³C4. The lower carbon isotope values of methane (δ¹³C1< δ¹³C2) may be attributed to thermal sulfate reduction (TSR), and the higher propane value (δ¹³C3> δ¹³C4) was caused by the mixing of the natural gases of the same genetic type formed at different stages. The methane hydrogen isotopes (δ²H-C1) of the natural gases in the Tarim Basin are in the range of −195‰∼-122‰ with an average value of −156‰. The ³He/⁴He ratios of the natural gases are in the range of 2.3 × 10⁻⁸∼66.8 × 10⁻⁸, suggesting that there is no deep mantle-derived gas in the platform area of the Tarim Basin. The carbon isotope fractionation of alkane gases (δ¹³C2<−28‰ and δ¹³C3<−25‰) indicates that the natural gas in the platform area of the Tarim Basin is oil-type. The modified plots of lnC1/C2vs. lnC2/C3 and C2/C3vs. δ¹³C2-δ¹³C3 of the natural gases indicate that the oil-type gas in the Tarim Basin has different predominant formation processes, including kerogen-cracking, oil-cracking, oil- and gas-cracking, secondary gas-cracking and mixing of the kerogen-cracking and oil-cracking gases. The gas evolved from gas-cracking gas, to oil- and gas-cracking gas and oil-cracking gas along the direction from the west side of the Manjar Sag to the central Tarim Basin. The natural gases in some areas of Tazhong are the mixtures of the kerogen-cracking and oil-cracking gases.
The origin and differential accumulation of hydrocarbons in the Cambrian sub-salt dolomite reservoirs in Zhongshen Area were studied based on comprehensive geochemical analysis of core samples, crude oil samples and natural gas samples. Mass spectrometric detection shows the core samples and crude oil samples are characterized by high C28 sterane content, low diasterane content, high gammacerane content and abundant aryl-Isoprenoids, and the associated gas has a low nitrogen content of 0.24%−4.02%, so it is inferred that the oil and gas are derived from Cambrian – Lower Ordovician source rock. The natural gas in the Middle Cambrian has a methane carbon isotope value of −51.4‰ − −44.7‰ and dryness coefficient of 0.65−0.78, representing associated gas, and the natural gas in the Lower Cambrian has a methane carbon isotope value of −41.4‰ − −40.6‰, and dryness coefficient of 0.99, representing cracking gas. The deep formations in the Tarim Basin contain cracking gas with high H2S content produced by thermo-chemical sulfate reduction (TSR). Due to the poorer reservoir properties and undeveloped fracture network system, the Middle Cambrian reservoirs have low charging degree of this kind of gas, so low H2S content (0.003 8%−0.200 0%); in contrast, with good reservoir properties and developed fracture network system, the Lower Cambrian reservoirs have a higher charging degree of this kind of gas, and thus high H2S content of 3.25%−8.20%. In summary, the oil and gas of Cambrian sub-salt dolomite reservoirs in Zhongshen Area are derived from Cambrian – Lower Ordovician source rock, and the differential accumulation of gas is the joint effect of reservoir physical property and development degree of fracture network system.
Analysis based on outcrop and drilling data indicates that source rocks can be formed in different tectonic settings. Transgressive process is benefit for source rocks development in various tectonic environments. Source rock development environment is increasingly limited from basin stretching to stable subsidence and finally to returning stage. The extensive transgressive overlap in stretching process is most favorable for the deposition of source rock in a large area. Source rocks are mainly distributed in the platform edge shelf and internal depression in stable subsidence stage. The closed environments such as intra-platform depression and regions behind high energy facies are favorable for source rock development in the returning stage. Sedimentary sequence and tectonic evolution show that the Cambrian-Ordovician source rocks in Tarim Basin experienced a whole process from basin stretching to returning stage, i.e. basin stretching in the Early Cambrian, stable subsidence in the Middle Cambrian to Middle Ordovician, and tectonic return in the Late Ordovician to Silurian. So that, 3 types prototype basins were formed. Stretching process in the Early Cambrian was the best period for source rock development. The northern depression, shelf in the southern basin edge and basin facies were the best zones. The basin showed difference in east and west during the Early and Middle Ordovician, and platform edge and limited basin developed in Manjiaer depression Aman transitional belt, being favorable for the development of source rocks. In the Late Ordovician, besides the high energy belt on the platform edge, Awati Depression and Aman transitional belt were intra-platform depressions, where limestone was deposited and was favorble for source rock development. Therefore, the major source rocks at different positions have different characteristics. ©, 2014, OIL & GAS GEOLOGY Editorial Board. All right reserved.
Based on comprehensive geophysics and geology data, the characteristics and reservoir controlling factors, as well as oil and gas distribution law and exploration direction, are analyzed for the Ordovician weathered crust in the Maigaiti slope and its periphery with paleo-uplift background in the Tarim Basin. Structural mapping and analysis reveal that the largest carbonate weathering crust is developed around the Maigaiti slope which forms a favorable reservoir-seal assemblage in combination with the overlying Silurian/Carboniferous mudstone. The Ordovician carbonates experienced multi-stages of karstification and are dominated by small-size fracture-vug reservoirs which are controlled by the karst paleogeomorphology, tectonism and lithology. Drilled wells are located mainly in the northern karst depression which has weak karstification and heavy filling. Paleogeomorphology reconstruction and seismic reservoir prediction suggest that reservoirs are well developed in the southern karst slope. The comprehensive analysis of structural revolution and hydrocarbon generation history demonstrates that the hydrocarbon migration and accumulation are controlled by the evolution and migration of the paleo-uplift. There are two charging stages: large scale oil accumulation in the late Hercynian period and adjustment and re-accumulation of oil cracked gas in the late Himalayan period. The Manan and Maixi slopes in the eastern and western sides of the paleo-uplift are the favorable areas for hydrocarbon accumulation and adjustment over a long period of time, have the conditions of forming large-scale carbonate fracture-vug type pools, and are important strategic relay exploration areas in the Tarim Basin.
Considering its vertical distribution characteristics, this article argues that the Lower Palaeozoic source rock in the Manjieer Sag is composed of three sets of source rocks of Middle-Lower Cambrian, Middle-Lower Ordovician, and Upper Ordovician. The source rock of the Middle-Lower Ordovician Heituwa Formation has similar sedimentary facies and developmental features to the Middle-Lower Cambrian source rock, and has abundant organic material. The Upper Ordovician source rock is poor and limited in distribution. The source rocks have different thermal evolution histories. In central and western Manjiaer sag, Middle-Lower Cambrian source rock entered oil generation peak in Late Caledonian and Early Hercynian, Middle-Lower Ordovician source rock in Late Hercynian, and Upper Ordovician source rock in Late Yanshan and Himalayan. The threefold division of the source rock is the foundation of detailed study on the derived source and the accumulation process of marine oil in Tarim platform area.
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