东营凹陷北带古近系碎屑岩储层长石溶蚀作用及其物性响应

Feldspar dissolution and its impact on physical properties of Paleogene clastic reservoirs in the northern slope zone of the Dongying sag

长石溶蚀作用是东营凹陷北带古近系沙河街组四段—三段中亚段碎屑岩储层成岩过程中重要的成岩事件。储层中长石颗粒次生溶孔普遍存在,镜下微观特征表明长石溶蚀越强烈,溶蚀产物越富集,因此,长石溶蚀能否有效改善其储层物性值得商榷。以普通和铸体薄片为基础,利用人-机交互图像分析技术,通过面积百分含量法,精确统计了长石溶蚀量和溶蚀产物含量;结合储层物性、流体包裹体及储层孔隙结构等资料,系统分析了长石溶蚀作用和溶蚀产物特征、长石溶蚀成岩环境及长石溶蚀对储层物性的影响。研究表明,东营凹陷北带沙河街组四段—三段中亚段储层中,长石溶蚀作用发生在相对封闭的高温成岩环境中,由于流体在孔隙网格系统中的流动速度低,且高温条件下长石溶蚀速度较快,因此,长石溶蚀产物并不能被有效带出溶蚀区或进行长距离搬运,而是在原地或准原地发生沉淀,即长石溶蚀只是将孔隙结构较好的原生孔隙进行重新分配,转换为基本等量的长石颗粒次生溶孔和高岭石(伊利石)晶间孔的过程;长石溶蚀后储层孔隙度变化不大,但孔隙结构变差,渗透率降低。

Feldspar dissolution is an important diagenetic event in the diagenesis of Es4-middle part of Es3 clastic reservoirs in the northern slope zone of the Dongying sag. Secondary dissolved pores of feldspar grains are almost ubiquitous in clastic reservoirs, however, microscopic investigations indicate that the stronger the feldspar dissolution, the richer the dissolution products, thus, whether physical property of a reservoir can be effectively improved though feldspar dissolution remains debatable. Based on matched thin sections and molded thin sections, we accurately counted amounts of both feldspar dissolution and dissolution products of rock thin sections using area percent method and humar~ computer interaction image analysis technique. Combined with reservoir property, liquid inclusion testing and pore structure data matched with rock thin sections, we systematically analyzed characteris- tics of feldspar dissolution and dissolution products, diagenetic environment of feldspar dissolution and its impact on physical properties of Es4-middle part of Es3 clastic reservoirs in the northern slope zone, Dongying sag. The result showed that feldspar dissolution in the studied reservoirs occurred in a relatively closed and high temperature diagenetic environment, dissolution prod ucts could not be effectively brought out the dissolved area or moved in a long distance due to a low speed of fluids in the pore net- work system and a quicker dissolution of feldspars under high temperatures, instead, they precipitated only in situ or almost in situ. In other words, feldspar dissolution just transformed primary intergranular pores with better pore structures to secondary pores of feldspar grains with an almost identical volume but poorer pore structures as well as to kaolinite （illite） intercrystal pores. Before and after dissolution, reservoir porosity changed little, but reservoir structure became poorer after dissolution and so was res- ervoir nermeabilitv.