敦煌盆地五墩凹陷中侏罗统中间沟组储层致密化过程分析

Analysis of reservoir densification process of Middle Jurassic Zhongjiangou Formation in Wudun Sag, Dunhuang Basin

五墩凹陷中侏罗统中间沟组储层岩性复杂,以砂岩、砂砾岩为主。通过区内钻井岩心资料的岩石薄片、扫描电镜、X-衍射和包裹体测温等分析化验资料,结合镜质体反射率、粘土矿物变化和各种成岩现象,重点分析储层特征和成岩作用,定量研究了中间沟组储层的致密化过程,明确储层致密与油气成藏的关系。研究表明,五墩凹陷中间沟组砂岩为长石质岩屑砂岩、长石岩屑砂岩和岩屑质长石砂岩,砂岩颗粒间磨圆以次棱角—棱角状为主,分选差,以点接触和点—线接触为主,砂岩成分成熟度和结构成熟度均偏低;储层物性整体较差,以特低孔、特低渗—超低渗为主;中间沟组储层主要经历了压实、胶结、溶蚀、交代和破裂等成岩作用,其成岩演化主体进入中成岩阶段A期。中间沟组储层致密化过程可以分成4个阶段,储层的原始孔隙度为33.35%,同生成岩期与早成岩阶段为压实孔隙骤减阶段,地壳的快速沉降造成原生孔隙锐减为12.8%。中成岩A1阶段,压实—压溶作用继续但强度减弱,胶结作用导致孔隙度进一步降低至7.84%,该阶段为胶结作用孔隙缓减阶段,此时储层为特低孔的致密储层。中成岩A2阶段,优质烃源岩大量生烃成藏,长石发生溶蚀而产生次生孔隙,为溶蚀作用孔隙微增阶段,孔隙度提高至9.44%。中成岩A2晚期发生胶结作用、交代作用和劈裂作用,多种作用导致孔隙缩减至7.5%。油气主成藏期为早白垩世晚期,此时储层已致密,因此中间沟组油藏为先致密后成藏,边成藏边增孔扩渗。

The reservoir lithology of Middle Jurassic Zhongjiangou Formation in Wudun Sag is complex, mainly sandstone and glutenite. Based on the analysis and test data of rock section,scanning electron microscope, X-ray diffraction and inclusion temperature measurement of drilling core data in this area, combined with vitrinite reflectance, clay mineral changes and various diagenesis phenomena, the reservoir characteristics and diagenesis are emphatically analyzed, and the densification process of the reservoir in Zhongjiangou Formation is quantitatively studied, and the relationship between reservoir compactness and oil and gas accumulation is clearly defined. The research shows that the sandstones of Zhonggangou Formation in Wudun Sag are feldspathic lithic sandstone, feldspar lithic sandstone and feldspar lithic Shi Ying sandstone. The grinding of sandstones is mainly sub-angular-angular, with poor sorting, mainly point contact and point-line contact, and the composition maturity and structure maturity of sandstones are low. The overall physical properties of the reservoir are poor, with ultra-low porosity, ultra-low permeability and ultra-low permeability as the main ones. The reservoir of Zhongjiangou Formation mainly experienced diagenesis such as compaction,cementation, dissolution, metasomatism and rupture, and its diagenetic evolution subject entered the middle diagenetic stage A. The densification process of the middle ditch formation reservoir can be divided into four stages. The original porosity of the reservoir is 33.35%. The syngenetic and early diagenetic stages are the stage of rapid reduction of compacted pores, and the rapid subsidence of the crust causes the primary pores to decrease sharply to 12.8%. In the middle diagenetic stage A1, the compaction-pressure solution continued but the strength weakened, and the cementation further reduced the porosity to 7.84%. This stage is the cementation pore slow-down stage, at which time the reservoir is a tight reservoir with ultra-low porosity. In the middle diagenetic stage A2, a large number of high-quality source rocks generate hydrocarbon and pool, and feldspar corrodes to produce secondary pores, which is the stage of micro-increase of dissolution pores, with the porosity increasing to 9.44%. In the late stage of diagenesis A2, cementation, metasomatism and splitting occurred, and the porosity was reduced to 7.5%. The main accumulation period of oil and gas is the late Early Cretaceous,when the reservoir is already dense, so the oil reservoir of the Middle Ditch Formation is first dense and then accumulated, and the pores are increased and the permeability is expanded while accumulating.