超浅层油藏成岩特征及对油气成藏的影响——以准噶尔盆地春风油田为例

春风油田位于准噶尔盆地车排子凸起东北部,其主力产层埋深平均500 m,属于典型的超浅层油藏。通过岩心描述与薄片观察,认为该油藏自下而上可分为底砾岩、粗砂岩—含砾砂岩、顶部细砂岩3套结构层。碳酸盐胶结为最典型的成岩现象,压实、溶解作用均较弱,胶结物主要在沉积初期形成,导致碎屑颗粒常呈漂浮状或点接触。3套结构层中的顶、底层分别由于强胶结和差分选导致储油能力差;中部有利结构单元内油水分布特征复杂,全区油水倒置现象严重。通过对该油藏进行成藏要素的解剖,认为沙湾组底部不整合、红车断裂带以及全区分布的厚砂层为油气成藏提供了良好的运移通道。但由于古构造、油气充注等因素的影响,北部、西北部以及西部地区早期发生了大规模碳酸钙沉淀,对后期运移的油气形成遮挡,造成了该区大规模油水倒置现象。研究区特殊的成岩现象及油藏分布特征充分说明,浅埋藏地区并非仅发育地层油藏和岩性油藏,受胶结作用影响也可形成一定规模的成岩圈闭油气藏,此类圈闭明显有别于常规的深层溶解型成岩圈闭。

Diagenetic characteristics of ultra-shallow reservoirs and influences on hydrocarbon accumulations:A case study of Chunfeng Oilfield,Junggar Basin

Located in the northeast of the Chepaizi Uplift of the Junggar Basin, the Chunfeng Oilfield is regarded as a typical ultra-shallow reservoir since the main producing strata are about 500 m deep. Core and thin section observations show that the reservoir can be divided into three structural layers: basal conglomerates, gritstones and gravel sandstones in the middle, and fine sandstones on the top. Carbonate cementation is the most typical diagenetic effect, which took place mainly early during deposition, so that clastic grains are usually floating or have point contacting. Accordingly, compaction and dissolution are weak. The oil storage ability of the top and the bottom layers is poor due to strong cementation and poor sorting respectively. The oil and water distribution of the middle reservoir is complex that oil/water inversion phenomenon is a serious concern. The studies of hydrocarbon accumulation show that the unconformities at the bottom of the Shawan Formation, the Hongche Fault Zone and the thick sandstones widespread in the study area provide good pathways for hydrocarbon migration. Influenced by paleo tectonics and hydrocarbon filling, large-scale calcium carbonate deposition took place in the north, northwest and west of the Chunfeng area during the early stage, which blocked later oil migration and resulted in the oil/water inversion phenomenon. The special diage-nesis and reservoir distribution made it clear that there is not only stratigraphic or lithologic reservoirs developed in the shallow region, but also a certain size of diagenetic traps developed under the influence of carbonate cementation. This special trap, different from the conventional dissolution diagenetic traps, provides for useful exploration potential in the northwestern margin of the Junggar Basin.