Sequence, sedimentary and their control over reservoir development of Lei4 Upper Sub-Member gas reservoir in Western Sichuan

Abstract The Leikoupo Formation marine facies gas pool in western Sichuan has a great exploration and development prospect, but the reservoir is characterized by a variety of lithologic types and lithologic combinations, and the distribution of the reservoir is complex. The previous research on the gas pool mainly focused on the analysis of hydrocarbon sources, reservoir conditions, source-reservoir cap-rock combination configuration, and controlling factors of karst reservoir formation, but few studies on sequence sedimentation as an organic whole and its influence about reservoir formation. In this paper, the sequence sedimentation and Leikoupo Formation 4 Member’s reservoir characteristics of the gas pool in west Sichuan are investigated by using core, thin section, SEM, physical properties, logging, and their effects on reservoir development are researched. The findings suggest the Leikoupo Formation 4 Member is separately composed of a 3 rd -order sequence, which includes three 4 th -order sequences. The Lei4 Upper Sub-Member, which is the target of gas pool, corresponds to the uppermost fourth-order sequence and is stably distributed in the research area with the thickness about 138-148 m. The 4 th -order sequence further includes a number of fifth- and sixth-order sequences. These high-frequency sequences are mainly shallow upward, with good horizontal correlation in cross-wells. The Lei4 Upper Sub-Member in the work area is generally controlled by marine carbonate tidal flat sedimentation. The intertidal subfacies are generally developed, which is a beneficial subfacies for reservoir formation. The lithology changes rapidly in the vertical direction, but is relatively stable in the horizontal distribution. The reservoirs lithology is complex, and various types of dolomite are developed. The reservoirs are mainly Class II and Class III (extra) low porosity - (extra) low permeability reservoirs characterized by thin interbedding and overlapping, with strong vertical heterogeneity. The (high frequency) sequence superposition style affected the distribution of favorable lithofacies and lithology such as laminar algal dolomite in the intertidal zone, which was conducive to the occurrence of favorable diagenesis such as dissolution and dolomitization during the penecontemporaneous period, thus controlling the development and spatial distribution of the reservoirs.