Research On Coal-related Gas Generating Process And Accumulation Periods In Qinshui Basin

Lots of petroliferous basins, are developed in Shanxi Province, where are the mainenergy mineral enrichment sites such as coal, oil and gas. The hydrocarbon potentialin most basins of Shanxi Province is great, while Qinshui Basin is one of the mostpromising areas for the oil and gas exploration.In this study, based on the collected data and sampling tests from Yangqu Blockand Zuoquan Block, the basin burial history and thermal evolution history wassimulated, and hydrocarbon generation and expulsion process in Qinshui Basin wasanalyzed. Through analysising the result of fluid inclusion of the two blocks, thecoal-related gas accumulated periods and characteristics were discussed, therefore,the conclusion of the coal-related gas potential layer was drawn.Through simulating the burial history and thermal evolution history of QinshuiBasin, the two-stage process for hydrocarbon generation and expulsion in QinshuiBasin is identified: the first phase is in Neopaleozoic--Triassic. In this stage, organicmatter experienced hypozonal metamorphism, and maturity increased slowly. In theTriassic period, with the formation sedimentation rate accelerated, the depth of coalmeasure strata increased rapidly to the maximum depth by the Late Triassic, thetemperature rising and the occurrence of the first phase of hydrocarbon generation;After Triassic, the basin was uplifted and eroded, temperature of coal formationdroped, and the organic hydrocarbon generation stoped; by the Late Jurassic-EarlyCretaceous, as well as the middle-late Yanshanian Movement, deep intrusive Rocksmaked temperature of coal formation increase, higher than temperature at the end ofTriassic period, the secondary hydrocarbon generation emergied. This period isconsidered as the major hydrocarbon generation period. After that formationcontinued to uplift, the paleotemperature gradually become normal, whilehydrocarbon generation was stopped.Combined with the evolutionary stage of hydrocarbon generation about coalmeasure strata in Qinshui Basin and its burial history, it is inferred that the first stageof hydrocarbon expulsion of the Qinshui Basin occurred in Early Jurassic. At that moment, coal measure strata had experienced the first stage hydrocarbon generation,the basin was uplifted and suffered denudation, and the pressure overlying thecoal-bearing strata was reduced, thus the oil and gas was desorbed and deliveried tothe adjacent reservoir. The second phase of hydrocarbon expulsion in Qinshui Basinoccured during the second stage of hydRocarbon generation, when coal-bearing strataproduced large amounts of coal-type gas. After gas content was saturated,hydrocarbon deliveried to the nearby reservoirs.Through analysing the fluid inclusions in Qinshui basin, it is found that the oil andgas in coal stratum of Qinshui Basin experienced two-step migration andaccumulation. The homogenization temperature histogram of hydrocarbon-bearingbrine inclusions coexistented with organic inclusions reveals two main peaks, andsalinity shows that inclusions are captured discontinuously. Reservoirs contains thefluid inclusions from the early to late stage of coalification. Fluid inclusions formed inthe early phase experienced secondary changes and generated carbonaceous bitumen.According to the analysis of the organic inclusions in the reservoirs of differentgeological age, its distribution is uneven in Taiyuan Formation and ShanxiFormation，with organic inclusions abundance from low–high, upward descending to<1%in the Shanghihezi Formation sandstone. For limestone reservoir, the abundanceof organic inclusions is very low, but highest in the Ordovician limestone cracks of allthe reservoirs.