张性盆地高温砂岩储层发育控制因素:以珠江口盆地文昌A凹陷为例

Controlling Factors of Reservoir Development under High Temperature in an Extensional Basin:A Case Study from Wenchang A Sag in Pearl River Estuary Basin

珠江口盆地西缘文昌A凹陷6号断裂带珠海组二、三段由于处于高地温环境,经历了较强的成岩改造,形成原生孔和次生溶孔比例相当的孔隙特征,发育低-中渗砂岩储层,平均孔隙度7.76%,而渗透率普遍小于2.5×10-3μm2。目前,该低渗储层形成的控制因素尚不清楚。通过物性测试、铸体薄片和扫描电镜等综合分析了储层低渗原因与优质储层发育控制因素。研究表明:文昌A凹陷珠海组二段埋深大于3 400m,发育扇三角洲前缘相,砂岩粒度粗,但成分成熟度偏低,且岩性变化大、泥质含量较高,造成抗压实能力相对较弱;其次,早期孔隙流体呈酸性,粒间缺乏早期碳酸盐胶结物,造成压实程度加大;绿泥石和伊蒙混层对喉道的堵塞也是该区低渗原因之一;该段中-粗砂岩储层经历了大气淡水溶蚀、有机酸溶蚀和热液溶蚀,其中晚期热液溶蚀对孔隙度增大有重要贡献。珠海组三段发育潮坪相,以特低渗储层为主,低渗的原因主要是砂岩粒度细,埋藏深,压实较强;其次,早期孔隙流体呈碱性,存在大量碳酸盐胶结物,以及局部层位石英次生加大强烈,造成特低渗;该段细砂岩原生孔隙较发育,其保存与绿泥石环边有关;由于强压实和绿泥石的堵塞作用导致喉道细小,孔喉半径比范围较大;高温强成岩背景下的低渗砂岩储层形成原因是该套砂岩储层分布预测的重要地质依据。

In Wenchang A Sag, western of the basin, the Zhuhai Formation sandstone reservoir has experi- enced intense diagenetic alteration, with high temperature. The pore assembly is composed of primary pores and secondary pores with similar percentage. The property of the reservoir is low-middlepermeabili- ty. The mean porosity is 7.76 %, and mean permeability is less than 2.5× 10-3 μm2, The controlling fac- tors for these low permeability reservoirs were still unknown. Through physical property measurement, observation of thin sections and SEM, the origin of low permeability and the controlling factors of high quality sandstone reservoir were analyzed. The facies of Zhuhai 2nd member in Wenchang Sag is fan-delta, with coarser grain of sandstone, lower compositional maturity, and more plastic grains, leading to weak capacity of compaction resistance. The pore-water is acidic in early diagenesis, resulted in lacking of car- bonate cement in early time, making the compaction stronger. The meteoric water, organic acids and hy- drothermal fluid dissolution in medium-coarse grain sandstone made the porosity increasing, in which hy- drothermal fluid dissolution has significant effect. Zhuhai 3rd member in Wenchang A Sag deposited in tid- al flat, with lower permeability than 2nd member. The factors included the fine grain, deeper buried, stronger compaction. And there were many carbonate mineral cement sandstone in early diagenesis, caused byalkaline pore water. The quartz overgrowth was intense, resulting in the porosity decreased and lower permeability. The high qualityreservoir of 3rd member was related to the chloride rim, enabling the inter- granular pores to be preserved. The stronger compaction of fine sandstone made throat tiny, and a big rati- o of pore vs. throat. The origin of low permeability is significant for the prediction of sandstone reservoir under high temperature and intense diagenetic alteration, which can guide further gas exploration.