渤海湾盆地渤中凹陷大气田形成条件与勘探方向

渤中凹陷是渤海湾盆地最富油、也最富气的凹陷,凹陷位于渤海海域中部,面积近9 000 km²,新生界厚度最大超过10 km,是渤海湾盆地面积最大、厚度最大的凹陷。渤中凹陷经历了古近纪断陷和新近纪区域拗陷两大阶段。古近纪断陷期双向伸展,形成"四洼一突"格局,断陷经历了由分隔洼陷到汇聚统一的演化过程;拗陷期持续沉降,形成古近系和新近系-第四系"双厚型"结构。渤中凹陷发育沙河街组三段、沙河街组一段及东营组三段3套优质烃源岩层系,优质烃源岩主要发育在沙河街组三段中-上部、沙河街组一段及东营组三段下部,上覆层巨厚,烃源岩演化程度高。基于统计分析重新构建了渤中凹陷不同层系烃源岩的生/排烃模型,从预测结果可以看出,渤中凹陷烃源岩排油量为512.5×10⁸m³、排气量为311.64×10¹¹m³。渤中凹陷潜山和孔店组储层油气源条件好,有近源特点。区域盖层为古近系下部厚层泥岩,成岩程度高、封堵能力强。潜山地层时代老、岩性硬、经历多期构造运动、裂缝发育、受强溶蚀、物性好、产量高,孔店组砂砾岩抗压实能力强。深层圈闭成群成带发育,规模大,勘探前景大,因此渤中凹陷深层具有形成大气田的条件,是发现大气田的有利领域。

Formation conditions and exploration direction of large gas field in Bozhong sag of Bohai Bay Basin

Bozhong sag is not only the most oil-abundant but also the most gas-abundant sag in Bohai Bay Basin, which lies in the middle of Bohai region with an area of nearly 9 000 km² and a thickness of Cenozoic strata more than 10 km. It is the largest and thickest sag in Bohai Bay Basin. Bozhong sag experienced two stages, i.e., Paleogene fault depression and Neogene regional depression. During the stage of Paleogene fault depression, a bi-extension occurred to form a pattern of "four subsag with one uplift". The fault depression experienced an evolutionary process from several separated subsags into a whole. Continuous sedimentation led to the formation of Paleogene and Neogene-Quaternary "double-thickness" structure during the depression stage. There are three sets of source rock layers in Bozhong sag, i.e., Members 1 and 3 of Shahejie Formation and Member 3 of Dongying Formation (Sha 1, Sha 3 and Dong 3). High-quality source rocks are mainly developed in the middle and upper part of Sha 3, and the lower part of Sha 1 and Dong 3. Since the overlying layers are ultra-thick, there occurred the high evolution of source rocks. Based on statistical research, this study has established the hydrocarbon generation and expulsion models for different layers of source rock in Bozhong sag. As viewed from the predicted results, the source rock in Bozhong sag has large oil and gas discharge of 512.5×10⁸m³ and 311.64×10¹¹m³, respectively. Reservoirs of Kongdian Formation and buried hill in Bozhong sag present good hydrocarbon source conditions and locate in the near-source zone. The regional caprock of deep reservoir is the thick mudstone in Dongying Formation with a high degree of diagenesis and a strong blocking ability. The buried hill formations are characterized by old age, hard lithology, multiple tectonic movements, fissure development, strong corrosion, favorable property and high yield. The glutenite in Kongdian Formation has a strong anti-compaction property. Large groups of deep traps are developed, showing great prospects for exploration. Thus, deep reservoirs in Bozhong sag have the conditions to form large gas field, which are favorable areas for the discovery of large gas fields.