尼日尔Termit盆地上白垩统储层岩石学特征及控制因素分析

本文对尼日尔Termit盆地17口井218个井壁取芯样品开展普通薄片、阴极发光、X衍射、扫描电镜及重矿物分析,研究上白垩统储层的岩石学特征,并探讨其控制因素。实验分析结果表明,Termit盆地上白垩统储层的岩石类型主要为石英砂岩,成分成熟度高,以石英为主,含量达86%以上,钾长石、方解石及斜长石等含少量,占比约5%,这种岩石类型对原生孔隙的保存比较有利;填隙物主要为粘土杂基,含量为7%左右,矿物成分以高岭石为主,利于对晶间孔的保留;岩石中发育硅质、方解石和铁质等胶结类型,胶结物的含量占比较小,对储层的破坏作用不大;重矿物类型为磁铁矿、赤铁矿、锆石、磷灰石,推测物源区的母岩类型主要为花岗岩;以细粒结构和少量的不等粒结构为主,分选中等-差,磨圆度一般为次棱角-次圆状,磨圆差。与国内石英砂岩沉积于高能的滨浅海环境不同,研究区石英砂岩成分成熟度高,但分选程度和磨圆度较低,一方面反映了物源区母岩富含石英,同时也反映了这种石英砂岩沉积于近源的河流-三角洲环境。影响研究区上白垩统储层物性的主要因素是岩石类型及成岩作用,由于储层的岩石类型为石英砂岩,压实作用对储层的破坏作用较一般岩石类型偏小。砂岩埋深小于2500m时,主要发育原生孔隙,面孔率大于15%。溶蚀作用对本区储层产生一定的影响,最常见的为长石的溶蚀,溶蚀作用形成粒内溶孔和粒间溶孔,还会扩大原生孔隙形成混合孔,改善了砂岩的储集性能。

The reservoir characteristics and controlling factors of the Upper Cretaceous sandstones in the Termit Basin, Niger

Petrography of the Upper Cretaceous sandstones of 218 side-wall coring samples from 17 wells was described by thin section, cathodoluminescence, X-ray diffraction techniques and heavy mineral analysis. Based on the result, we try to study the lithological characteristics of the Upper Cretaceous sandstones and explore the related factors. The results show that the Upper Cretaceous sandstones, having high component maturity, are quartzarenite with quartz percentage more than 86%. There is little content of potash feldspar, calcite and plagioclase in the rock with the content of about 5%. This kind of rock is more advantageous to the preservation of primary porosity. Pore-filling cements mainly consist of clay minerals with the content of about 7%, which are dominated by kaolinite. It is favourable to the preservation of intergranular pore. The cements are mainly siliceous, calcite and ferruginous. Because the Content of cement is little, the damage to the reservoir is not effective. Main heavy minerals include magnetite, hematite, zircon, apatite, indicating that the Upper Cretaceous sandstones may be source from granite rocks. With low texture maturity, mainly have fine grained texture and minor inequigranular texture. Grains are mainly characterized by point contact and subangular-subcircular rounding, with fair to poor sorting. It is different from domestic quartz sandstone that deposited in coastal environment; the quartz sandstone in the study area has high component maturity and low structural maturity. It Indicate that the source rocks may be rich in quartz, and the same time the sandstone deposited in the near source fluvial deltas. Cementation is the main factor that influences the Upper Cretaceous sandstones. Because main sandstones are quartzarenite, compaction has less negative effect on reservoirs which helps to maintain porosity of sandstones. The face rate is more than 15% if the buried depth of sandstone is less than 2500 meters. Dissolution, common in feldspar dissolution, has certain influence on the Upper Cretaceous reservoir. Dissolution produces intergranular and intragranular dissolved pores, and enlarges primary pores, which improves reservoir performance.