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Based on the analysis of the hydrocarbon geochemical characteristics in the Kuqa petroleum system of the Tarim Basin, this study discusses the causes and controlling factors of the phase diversities and their differences in geochemical features. According to the characteristics and differences in oil and gas phase, the petroleum system can be divided into five categories: oil reservoir, wet gas reservoir, condensate gas-rich reservoir, condensate gas-poor reservoir and dry gas reservoir. The causes for the diversities in oil and gas phases include diversities of the sources of parent material, maturity of natural gas and the process of hydrocarbon accumulation of different hydrocarbon phases. On the whole, the Jurassic and Triassic terrestrial source rocks are the main sources for the hydrocarbon in the Kuqa Depression. The small differences in parent material may cause diversities in oil and gas amount, but the impact is small. The differences in oil and gas phase are mainly affected by maturity and the accumulation process, which closely relates with each other. Oil and gas at different thermal evolution stage can be captured in different accumulation process. [ABSTRACT FROM AUTHOR]

Silurian sandstone in Tarim Basin has good reservoir properties and active oil and gas shows, especially thick widely-distributed bituminous sandstone. Currently, the Silurian was found containing both bitumen and conventional reservoirs, with petroleum originating from terrestrial and marine source rocks. The diversity of their distribution was the result of 'three sources, three stages' accumulation and adjustment processes. 'Three sources' refers to two sets of marine rocks in Cambrian and Middle-Upper Ordovician, and a set of terrestrial rock formed in Triassic in the Kuqa depression. 'Three stages' represents three stages of accumulation, adjustment and reformation occurring in Late Caledonian, Late Hercynian and Late Himalayan, respectively. The study suggests that the Silurian bitumen is remnants of oil generated from Cambrian and Ordovician source rocks and accumulated in the sandstone reservoir during Late Caledonian-Early Hercynian and Late Hercynian stages, and then damaged by the subsequent two stages of tectonic uplift movements in Early Hercynian and Pre-Triassic. The authors presumed that the primary paleo-reservoirs formed during these two stages might be preserved in the Silurian in the southern deep part of the Tabei area. Except for the Yingmaili area where the Triassic terrestrial oil was from the Kuqa Depression during Late Himalayan Stage, all movable oil reservoirs originated from marine sources. They were secondary accumulations from underlying Ordovician after structure reverse during the Yanshan-Himalayan stage. Oil/gas shows mixed-source characteristics, and was mainly from Middle-Upper Ordovician. The complexity and diversity of the Silurian marine primary properties were just defined by these three stages of oil-gas charging and tectonic movements in the Tabei area. [ABSTRACT FROM AUTHOR]

Abstract: The formation mechanism of deep effective reservoirs is elaborated based on the analysis of the conditions for forming natural gas accumulations in deep Kuqa Depression. Natural gas charging is closely related to the formation of effective sandstone reservoirs in the deep zone: reservoirs which capture oil and gas in the early period are of better quality during the long burial period; reservoirs which do not capture oil and gas in the early period is compact after deep burial because of compaction. Oil and gas charging before the reservoir is deeply buried significantly inhibits mechanical compaction and fracture closure as well as densification, so the effective pores and fractures will be preserved. The formation conditions of large gas fields in Kuqa Depression are as follows: sufficient coal gas source; great gas generation potential of high-evolution Jurassic source rocks; secondary pores and micro-fissures developed in deep sandstone reservoirs; wide distribution of gypsum-salt cap rock with large thickness; good association of source-reservoir-cap in space; well developed hydrocarbon migration pathways of dominantly faults; high expulsion efficiency; big migration and accumulation coefficient and deep or ultra-deep traps developed below salt structures. [Copyright &y& Elsevier]