川东北元坝地区上二叠统长兴组优质储层形成机制

FORMATION MECHANISM OF HIGH-QUALITY RESERVOIRS IN THE UPPER PERMIAN CHANGXING FORMATION IN YUANBA GAS FIELD,NORTHEASTERN SICHUAN BASIN

川东北元坝气田为千亿储量的海相气田,上二叠统长兴组礁滩相气藏具有埋藏深、高温、高压、高含硫化氢、气-水关系及工程地质条件复杂等特征。优质储层主要为礁滩相的白云岩,具有物性好、非均质性强、含气性好的特点。研究表明,台地边缘生物礁及台地边缘浅滩相控制了有利的储层岩石组合类型与分布,是优质储层发育的基础;白云石化作用和溶蚀作用改善和提高了储层岩石的孔隙度,构造活动形成的多期裂缝提高了储层的渗流性能,沉积-成岩-构造破裂等多因素的综合作用和合理配置是川东北元坝地区长兴组优质白云岩储层形成关键。

The Yuanba gas field located in the northwestern Sichuan Basin contains the original inplace gas volume of more than one hundred billion cubic meters identified in marine strata. The Upper Permian Changxing Formation as the main gas reservoir for the field is characterized by an abundance of H2 S gas,and is typically buried at deep depths where it is subjected to high temperatures and pressures,gas and water distribution systems are intricate and engineer geology conditions are complicated within it. The dolomites of reef and shoal facies in the Changxing Formation have been regarded as high-quality reservoirs, which exhibits high porosity and permeability,high reservoir heterogeneity,considerable gas production capacity. The aim of this study is to improve the understanding of the dolomite origin using the carbon and oxygen isotopes and the high-quality reservoir formation at deep and ultra-deep depths by analyzing strong correlations in reservoir petrography,depositional facies,diagenesis and fracture development system in detail. The research shows that platform-margin reef and shoal facies in the study area are the most favorable facies belts controlling high-quality reservoir formation because reservoir rock types and spatial geometry are generally determined by depositional characteristics. Dolomitization and dissolution is significantly helpful in increasing reservoir porosity. Multi-history fracture system appearing tectonic in originis very constructive to improve reservoir permeability. It is possibly fair to say that the highquality reservoir formation in the Changxing Formation was determined by synthetic configurations and dynamic interplays between multiple geological processes and factors.