准噶尔盆地玛湖凹陷南部风城组火山岩油藏特征与聚集模式

Characteristics and accumulation mode of volcanic rock reservoir of the Fengcheng Formation in the southern of Mahu Sag,Junggar Basin

为明确玛南斜坡区风二段顶部薄层风化壳火山岩油藏油气富集规律及主控因素,对研究区火山岩特征、油藏特征和油气富集特征进行了综合分析,建立了斜坡区火山岩的油气聚集模式。研究表明,玛湖凹陷南部风二段主要发育玄武岩和凝灰岩,其中火山溢流相的玄武岩发育较高渗透性的储集层,油气最为富集。原油整体属于中-高黏度、轻质-中质含蜡原油,其C_(20)、C_(21)、C_(23)三环萜烷分布呈山峰型,三环萜烷相对含量明显较高。原地烃源岩与原油相比,其三环萜烷分布呈上升型,生物标志化合物成熟度参数也显示出更低成熟度的特征,结合研究区原油碳同位素特征与风城组烃源岩一致的研究结果,认为研究区油藏受到多期的油气充注。研究区火山岩的垂向与横向非均质强,原油密度随深度增加而降低,认为油气主要沿火山岩之下的渗透性地层由低部位向高部位运移,在浮力作用下向上进入火山岩储集层中,并在储层物性相对较好,且保存条件良好的构造圈闭或岩性圈闭中聚集成藏,为典型的“斜坡区构造-岩性-多期火山岩油气藏成藏模式”。

[Objective]To clarify the oil and gas accumulation patterns and main controlling factors of the thin weathered crust volcanic rock reservoir at the top of Member 2 of the Fengcheng Formation in the southern Mahu Sag(Manan Sag).[Methods]This paper provides a comprehensively analyses of the volcanic rock characteristics,reservoir properties,and oil and gas geological features.Furthmore,an oil and gas accumulation model for volcanic rocks in the slope area is established.[Results]Research has shown that Member 2 of the Fengcheng Formation in the southern Mahu Sag is mainly composed of basalt and tuff.The basalt with the volcanic overflow phase developed high-permeability reservoirs abundant in oil and gas.Overall,the crude oil is medium-high viscosity and light-medium waxy.Its distribution pattern of C_(20),C_(21),and C_(23)tricyclic terpanes resembles a mountain peak,showing a significantly higher relative content of tricyclic terpanes.In contrast to crude oil,the distribution of tricyclic terpanes in in-situ source rocks demonstrates an upward pattern,with maturity parameters of biomarkers also displaying lower maturity characteristics.Based on the consistent research results of the carbon isotope characteristics of crude oil in the study area and in the Fengcheng Formation source rocks,it is infered that the oil reservoir in the study area has undergone multistage oil and gas charging.[Conclusion]The volcanic rocks in the study area exhibit strong vertical and horizontal heterogeneities,with crude oil density decreasing with depth.It is believed that oil and gas mainly migrate from low to high parts along permeable strata below volcanic rocks and enter volcanic rock reservoirs under buoyancy.They accumulate reservoirs in structures or lithological traps with relatively good reservoir properties and preservation conditions.This is a typical slope area-structure lithology-multistage volcanic rock oil and gas reservoir formation model.