琼东南盆地崖南凹陷崖城组辫状河三角洲沉积特征及油气地质意义

Depositional characteristics and petroleum geology significance of braided delta system in Yacheng formation,Yanan sag,Qiongdongnan basin

通过岩心成分、粒度及沉积构造特征综合分析,对琼东南盆地崖南凹陷西北部渐新统崖城组的沉积特征进行了研究。结果表明,研究区崖城组发育辫状河三角洲沉积,岩性以粗碎屑岩为主,夹多层薄煤层并含有海相自生矿物海绿石;粒度累积概率曲线类型多样,兼有牵引流和沉积物重力流成因特点;层理类型包括平行层理、沙纹交错层理、均质层理和潮汐层理等,此外还发育冲刷面及变形构造,反映了复杂多变的水动力环境。对测井曲线特征进行分析并结合前人研究成果,建立了研究区崖城组辫状河三角洲的沉积模式,主要发育辫状河三角洲平原和前缘亚相,为断陷早期平衡充填背景下发育于箕状断陷主断裂隐没端附近的海陆交互相沉积,物源主要来自海南隆起和崖城凸起。研究区辫状河三角洲沉积中的煤层可作为良好的气源岩,与以粗碎屑岩为主的储层组成了有利的生储配置,对崖城13-1气田的形成具有重要意义。

Focusing on the northwestern part of Yanan sag of Qiongdongnan basin, we studied the sedimentary characteristics of Yacheng formation in Oligocene mainly through composition, grain size and sedimentary structure analysis. Typical braided delta sedimentary system has been identified in Yacheng formation. The lithology of the braided delta system is characterized by coarse clastic rocks which contain several thin coal seams and marine authigenic minerals such as glauconite. The rocks have multiple grain size cumulative curves both characterized by tractive and gravity flow. Sedimentary structures include multiple beddings such as parallel, tidal, uniform beddings and climbing-ripple cross-lamination, and many erosion surfaces and deformations, reflecting the complex and changeable hydrodynamic condition. Assisted by the logging curve characteristics and previous works, we constructed the sedimentary model of the braided delta system. We suggest that the braided delta system of the Yacheng formation in the study area was formed in the setting of balance filling in the early forming stage of the basin and developed near the vanish end of the main fault of the half gra ben. The braided delta system mainly includes subfacies of braided delta plain and braided delta front, and the main source areas are the Hainan uplift and Yacheng heave. Coal seams of the braided delta system are high quality source rock, together with the coarse grain rocks, forming the favorable lithological assemblage, which is important to the formation of the Yacheng13-1 gas field.