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Abundant organic inclusions are present in the Qinglong antimony deposit. However, the source rocks of these organic matters have not been reliably identified. Recently, a paleo-oil reservoir was found in the Qinglong antimony deposit. In view of similar components of gaseous hydrocarbon, we propose that the organic matters observed in inclusions in Qinglong antimony deposit would come from this paleo-oil reservoir. We used the Re-Os dating method to determine the age of the bitumen from this paleo-oil reservoir, and obtained an isochron age of 254.3±2.8 Ma. The age indicates that the oil-generation from source rock occurred in the early Late Permian, earlier than the Sb mineralization age (∼148±8.5 Ma) in the Qinglong antimony deposit area. After oil generation from Devonian source rock, first and secondary migration, the crude oil have probably entered into the fractures and pores of volcanic rocks and limestone and formed a paleo-oil reservoir in the western wing of Dachang anticline. As burial process deepened, the crude oil has turned into natural gas, migrates into the core of Dachang anticline and formed a paleo-gas reservoir. The hydrocarbons (including CH4) in the reservoirs can serve as reducing agent to provide the sulfur required for Sb mineralization through thermal chemical reduction of sulfates. Therefore, the formation of oil-gas in the area is a prerequisite for the Sb mineralization in the Qinglong antimony deposit. [ABSTRACT FROM AUTHOR]

The unique Moxi–Gaoshiti Bulge of Sichuan Basin in southwestern China provides insights into the biomarker signatures of bitumens and their geological significance for paleo-oil reservoirs in Precambrian strata. Solid bitumens with various shapes occur widely in the voids, solution pores, cracks, stylolites, and veins of reservoir rocks. Biomarker analysis was conducted on bitumens and the associated source rocks by quantitative detection with gas chromatography–mass spectrometry (GC–MS) to constrain thermal maturity, sedimentary environments, bitumen/paleo-oil sources, and the possible paleo-oil filling directions, which are dominant geological factors that control paleo-oil accumulations. Quantitative analysis of biomarkers showed that the bitumens in the Sinian dolomite of the Moxi–Gaoshiti Bulge belong to the same group. The bitumens are highly matured, and their reflectance values (%R b ) are greater than 1.5 with an average of 2.96. These bitumens originated from thermal cracking of oil (pyrobitmen) via a high-temperature process. The biomarker characteristics revealed that the bitumens and related paleo-oil reservoirs originated from a highly reduced but moderately saline marine shale environment. The marine shales of the Sinian Dengying and lower Cambrian Qiongzhusi formations are their major source rocks. The directions of the primary oil-filling pathways in the Gaoshiti and Moxi sub-bulges are from west to east and from northwest to southeast, respectively. These directions indicate that the Deyang–Anyue sag west of the Moxi–Gaoshiti Bulge is the major hydrocarbon source kitchen. [ABSTRACT FROM AUTHOR]

Abstract: The formation and evolution of Silurian sandstone reservoirs, which are characterized by the coexistence of reservoir bitumen with different types of commercial oil pools in a very extensive area around the slope and uplift areas of the Manjiaer Depression in the Tarim Basin, NW China, are still in question. In this paper we report our investigations on the geological and geochemical characteristics of this reservoir. The results show there are four types of oil shows in the reservoir: severely biodegraded solid bitumen, biodegraded heavy oil, normal oils and light oils with slight biodegradation. Three suites of petroleum inclusions were identified in the reservoir which were formed during the early Devonian, Triassic to Cretaceous and late Tertiary, respectively. Petroleum in the Silurian sandstone reservoirs mainly originated from the mid-lower Cambrian source rock and mid-upper Ordovician source rock. The thermal maturity evolution of the source rocks in slope areas has shown that there were three main stages of oil generation that controlled the formation of the above three stages of petroleum inclusions. This evidence indicates that the Silurian reservoir has experienced three petroleum charge events during its geological history. The first charge event occurred during the early Devonian and it resulted in giant paleo-pools, which gradually evolved into the presently widespread bitumen due to the combination of biodegradation and thermal alteration. The second charge covered a long period from Triassic to Cretaceous and probably was related to both the mid-lower Cambrian source rock and the mid-upper Ordovician source rock. The petroleum generated during this stage mixed and interacted with the low matured residual bitumen and eventually formed the present soft bitumen and some residual heavy oils in the reservoirs. The third charge event, mainly associated with the mid-upper Ordovician source rocks, occurred mainly during late Tertiary and accounted for most of the present commercial petroleum pools (mainly normal and light oils). [Copyright &y& Elsevier]

The Ordovician buried hill reservoir in the western Lunnan area, a type of dissolved fracture and cavernous reservoir, is mainly composed of heavy oil. The oil is the mixture sourcing from the Middle-Lower Cambrian and Middle-Lower Ordovician, with three stages of pool forming process: (1) the destruction and parallel migration/accumulation during the late Caledonian to early Hercynian; (2) the oil and gas accumulation during the late Hercynian characterized by adjustment upward along faults and parallel migration/accumulation; (3) the formation of heavy oil during the latest Hercynian. The Ordovician buried hill reservoir is affected by the diffusion of light oil and gas but had no hydrocarbon charging during the late Yanshan period to Himalayan period, but in this period, formed the association of heavy oil and dissolved gas cracked from crude oil with dry coefficient of 0.91-0.96. The study on accumulation process of the Ordovician buried hill reservoir has important implications for the exploration potential of early oil and gas accumulation in the cratonic area of the Tarim Basin. [ABSTRACT FROM AUTHOR]

Abstract: Silurian sandstone reservoirs in the Tazhong Uplift in the Tarim Basin, northwestern China are rich in bitumen and movable oil. Analysis of the biological markers indicates that the bitumen and movable oil came from two source rocks in the Cambrian and Middle/Upper Ordovician system, both of which are belong to marine facies. The Silurian system experienced three stages of oil filling, namely Late Caledonian period (400Ma), Late Hercynian period (240Ma) and Yanshan-Himalayan (40Ma) period. Most of the oil emplaced in the Late Caledonian was subsequently oxidized to bitumen. The movable oil found at the present time was mainly generated from the Middle/Upper Ordovician source rocks, with only a minor contribution from the Cambrian source rocks generating in the two later periods. Bitumen and heavy crude were largely trapped below a red mudstone unit while movable oil was concentrated below an underlying gray mudstone unit (the middle sub-section of bitumen sandstone section). Lateral migration into the Silurian reservoirs took place from northwest to southeast in each stage and was controlled by an inherited palaeohigh. While individual relatively limited in extent, the tidal-flat sandstones of the main reservoir have stacking patterns that generate extensive sand sheets. Faulting can both enhance connectivity and create traps. [Copyright &y& Elsevier]