埋藏条件下碳酸盐岩储层孔隙充填与保存机理研究—以鄂尔多斯盆地东南部下古生界岩溶型储层为例

Paleokarst reservoir can form large scale-super large scale oil and gas field, therefore it plays a significant role in oil and gas exploration. Extensive studies have been done on paleokarst reservoir, mainly on the reservoir characteristics, origin and geomorphic. However, few studies have been done on the carbonate minerals formation mechanism filled in pores near to paleokarst interface. In the long burial diagenesis, large amount of pores formed by paleokarst are filled, which reduce the reservoir property. How the pores are preserved in buried environment may be the key factors affecting reservoir development. Based on this, the project aims to study the carbonate minerals filled in pores in lower palaeozoic Majiagou formation in Yichuan area of southeast Ordos basin. With petrology, mineralogy, isotope and element geochemistry as main study method, in combination with burial history of the study area and other diagenesis process, the project aims to find out the formation mechanism of carbonate minerals filled in pores and the porosity preservation mechanism to provide reference for the study of development and reserving of paleokarst reservoir and the exploration of related reservoir. Yican 1 well is drilled in southeast Ordos basin in 2015 and break through has been made in Ordovician Majiagou formation, which provide opportunity for the study of fluid property, pores filling and preservation mechanism near to paleokarst interface.

古岩溶储层常形成大型-超大型油气田，因而在油气勘探中占据重要地位，前人对此进行了大量研究，但主要集中于对岩溶储层特征、成因和岩溶古地貌的研究，有关储层孔隙充填及保存机理研究较少。在漫长的埋藏成岩演化过程中，表生期形成的孔洞缝很大一部分发生了充填作用，降低了储层的储集性能，孔隙如何被有效地保存下来，可能是储层发育的关键。基于此，本项目拟以鄂尔多斯盆地东南部宜川地区下古生界马家沟组岩溶型储层孔隙充填的碳酸盐矿物为主要研究对象，以岩石学、矿物学、同位素和元素地球化学为主要研究方法，结合埋藏历史及成岩过程研究，查明封堵岩溶界面附近孔洞缝的碳酸盐矿物的形成机制，为古岩溶储集空间的发育与保存研究、以及具有相应成因机制储层油气勘探提供可借鉴的基础资料。2015年在鄂尔多斯盆地东南部实施了参数井宜参1井，在奥陶系马家沟组获得突破，为岩溶型储层孔隙充填碳酸盐形成流体性质、充填及保存机制研究提供了契机。

孔洞充填作用是控制储层发育的重要因素，鄂尔多斯盆地东南部奥陶系马家沟组膏溶孔多为白云石、方解石、黄铁矿、石英等矿物充填，充填率达90%以上，孔隙体积显著减小。而盆地中东部靖边气田则很少被充填，充填率小于70%，具有良好的储集空间。研究表明，硬石膏溶蚀孔中充填的方解石具有相对负偏δ18O值（-15.58‰~-8.96‰VPDB）和δ13C值（-7.56‰~0.26‰VPDB），可能是热化学硫酸盐还原作用形成。方解石原生包裹体具有极高的均一化温度（140 ~ 234℃）和较高的盐度(19.13 ~ 23.18 wt.% NaCl)证实了上述解释。TSR的副产物白云石含量仅次于方解石，在方解石沉淀的条件下，孔隙水中的Mg2+/Ca2+比值增加促进白云石的形成。黄铁矿是由TSR释放的H2S与地层中Fe2+反应形成的，因此，具有立方体形态、较高的δ34S值 (10.50‰~24.00‰VCDT)。石英的均一温度较高（113-154℃），被认为是在TSR后从方解石和黄铁矿中析出的低PH溶液中析出的。鄂尔多斯盆地东南部的古地理位置远低于靖边气田，来源于东南方向的海水使其长期淹没在海洋潜水环境中。由于孔隙水中富集的硫酸钙发生TSR作用促进白云石、方解石、黄铁矿和石英等矿物充填于铸模中，造成研究区孔隙度和渗透率较低。