Shiyou shiyan dizhi (May 2021)
Exploration potential and favorable direction of tight gas in Middle Jurassic Xiashaximiao Formation in Sichuan Basin
Abstract
Tight gas is an important gas-pool type for natural gas resource and output increment in China. The Middle Jurassic Xiashaximiao Formation (J2x) is an important target for the exploration of tight oil and gas in the Sichuan Basin. Based on decades of exploration practice and studies on accumulation conditions of tight gas, it was indicated that the J2x formation gas reservoir is a secondary structural lithologic gas reservoir with multiple layers, and was constrained by source, position and sedimentary facies. The major characteristics of these reservoirs are (1) the dark mudstones, carbonaceous mudstones and coal seams in the Lower Jurassic Ziliujing and Upper Triassic Xujiahe formations provided abundant natural gas sources; (2) the mud-packed sand structure formed by the combination of various channel sand bodies and shale in sedimentary systems such as alluvial fans and deltas was favorable for the enrichment of natural gas; (3) the widely distributed "estheria shale" or purple-red mud shale layer acted as regional cap rock; (4) three major paleo-uplifts and three major depressions formed after the Himalayan Movement, developed a series of structures on the top of the Xiashaximiao Formation, which was favorable for the formation of gas-bearing traps distributed throughout the basin and (5) the faults from the Upper Triassic Xujiahe Formation to the Xiashaximiao Formation provided migration channels for far-source gas reservoirs, while gas in the Ziliujing Formation could migrate to near-source reservoirs through small faults or micro-fractures. Thus a tight gas accumulationmodel of the Xiashaximiao Formation was established. The Xiashaximiao Formation possesses the foundation for forming large and medium-sized gas fields. The western part of Sichuan Basin has the largest potential for gas exploration and followed by the northern and southern parts. In the central Sichuan Basin, the reservoir-forming conditions are poorly configured and made its exploration potential poor.
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