碳酸盐岩台地边缘沉积结构差异及其油气勘探意义——以川东北早三叠世飞仙关期台地边缘带为例

随着研究的深入,逐渐认识到碳酸盐岩台地边缘带(简称台缘带)具有沉积结构和演化的差异性,其不但记录了古地理格局及其演化过程,同时也对相关油气储层的发育具有重要的影响。以四川盆地东北部早三叠世飞仙关期同期近平行的三排台缘带为研究对象,通过钻井、野外露头及地震等资料综合分析,揭示研究区早三叠世飞仙关早期台缘鲕粒滩带明显继承了晚二叠世长兴期台缘礁滩带的分布,并具有整体向东迁移的特点。不同台缘带之间具有明显差异,可识别出迁移型和加积型两种台缘结构样式,迁移型台缘带主要分布在元坝-龙岗-梁平台缘带(简称1号台缘带)、简池-鸡王洞-沙陀台缘带(简称3号台缘带),其具有台缘斜坡坡度缓、迁移明显,造成台缘带鲕粒滩整体分布宽但厚度薄、横向间互云化的特点;加积型台缘带主要分布在铁厂河-普光-罗家寨台缘带(简称2号台缘带),沉积结构总体上表现为加积特征,但晚期具有向东迁移特点,台缘带窄、台前滑塌普遍,鲕粒滩分布集中且厚度大、云化彻底。不同台缘带内部沉积和储层分布具有差异性,1号台缘带内部具有斜坡带陡缓相间,缓坡区台缘带鲕粒滩迁移明显,鲕粒滩累积厚度薄且云化弱或不发育,相对的陡坡区迁移幅度略小,鲕粒滩累计厚度略厚,且具有向上云化程度逐渐增强的特点;由于面对相对开阔的海域,3号台缘斜坡带风暴影响明显,具有横向上的云化间互及垂向云化增强特征明显,资料有限,横向地貌变化尚待进一步揭示;2号台缘带具有横向上的鲕粒滩厚薄变化,但加积和整体云化特征稳定。台缘沉积结构差异性明显受早期长兴期古地貌、海平面变化及古风向和沉积期差异沉降等因素的联合影响。研究建立了相关台缘带沉积和白云岩分布模式。

Sedimentary texture diversity of different carbonate platform margins and it its significance for petroleum exploration: A case study of carbonate platform margins in Feixianguan period of the Early Triassic, NE Sichuan basin, China

As research continues, the diversity of sedimentary textures and evolution about carbonate platform margin belts (abbreviated as platform margin belt) have been gradually realized, which not only records the palaeogeographic characters and its evolution process, but also significantly influenced on its carbonate oil and gas reservoir. Three parallel distributed Early Triassic Feixianguan period carbonate pateform marginal belts, which located in NE Sichuan basin, were studied to reveal the distribution of carbonate platform margin belts. Through the comprehensive analysis of well drilling, outcrops and seisimic data, the study verified that the distribution of the platform marginal reef shoal zones in the Late Permian Changxing period was obviously inherited by the platform marginal oolitic shoals in the Early Triassic Feixianguan period, and the distribution of oolitic shoals migrated toward east during the Early Triassic Feixianguan period. Results showed that there are obvious differences of texture between these three platform margin belts. Migration and aggradation, the two kinds of platform margin texture, can be identified obviously in different platform margins. Migration platform margin belts include Yuanba-Longgang-Liangping platform margin belt (abbreviated as No.1 platform margin belt) and Jianchi-Jiwangdong-Shatuo platform margin belt (abbreviated as No.3 platform margin belt), with typical characteristic of gentle platform-margin slope and obvious migration, resulting in wide and thin oolitic shoal developing, and mutual dolomitization along the carbonate platform margin. Aggradation platform margin belt distributs in Tiechanghe-Puguang-Luojiazhai platform margin belt (referred to No.2 platform margin belt), which is characterized by aggradational sedimentary texture, and eastward migration at the later period. Aggradation platform margin belt general is always narrow with front slump, the oolitic shoal in aggradation platform margin belt is concentrated distribution with a great thickness and complete dolomitization. The distribution of sedimentation and reservoir is difference from each other in different platform margin oolitic shoals. The gradient of the No.1 platform margin is alternation between gentle and steep, and the characteristics of oolitic shoals in gentle platform margin area are thin cumulative thickness, weak dolomitizaiton or non-dolomitizaiton, and obvious migration. However, oolitic shoals in steep platform margin area has the features of short distance migration, massive cumulative thickness and dolomitizaiton gradually enhancing upward. As facing relative open sea, influenced by storm obviously, the No.3 platform margin is characterized by lateral dolomitization among layers and vertical dolomization enhancements. Due to the absence of supporting data, geomorphy features of the No.3 platform margin belt need further researched. The thickness of oolite shoals in the No.2 platform margin belt changes in transverse direction, but its aggradation texture and overall dolomitization is relatively stable. The diversity sedimentary texture in different platform margin belts at study area are influenced obviously by comprehensive factors, which includes palaeogeomorphology of Late Changing period, sea-level change, palaeowind direction and synsedimentary differential sagging during Feixianguan period. In this paper, sedimentary and dolomite distribution model of platform margin belts during the period of Lower Triassic Feixianguan Formation has been reconstructed.