玛湖凹陷大型浅水退覆式扇三角洲沉积模式及勘探意义

Sedimentation Model and Exploration Significance of Large-Scaled Shallow Retrogradation Fan Delta in Mahu Sag

玛湖凹陷及其周缘地区上二叠统上乌尔禾组—下三叠统百口泉组为一套坳陷盆地背景下近物源的粗粒沉积物,岩心上可识别出13类岩相、7种沉积微相,是在平缓斜坡背景下的大型浅水退覆式扇三角洲沉积,在凹陷斜坡区发育多个扇三角洲,各扇体之间相互搭接连片。该类大型浅水退覆式扇三角洲的形成与3个地质条件密切相关,一是沉积时持续隆升的周缘老山、稳定的水系为各大扇体提供充足的物源;二是具备盆大、水浅、坡缓的良好古地理背景;三是持续湖侵、多级坡折,是大面积砂体叠加连片的一个重要条件。建立了大型浅水退覆式扇三角洲沉积模式,有效指导了凹陷区油气勘探重大突破与储量快速落实。

The upper Wuerhe formation of Permian and Triassic Baikouquan formation in Mahu sag and its adjacent areas are a set of near-source coarse deposits under the background of depression basin that are large-scaled shallow retrogradation fan-delta deposits under thebackground of gentle slope, and 13 lithofacies and 7 sedimentary microfacies are identified based on core observation. Multiple fan deltasare developed on the slope of the sag, and the fan bodies connect with each other. The formation of this kind of large-scaled shallow retro-gradation fan delta is closely related to 3 geological conditions. The first is that continuously uplifting periphery ancient mountains and sta-ble river systems provided sufficient provenance for each fan during deposition; the second is that there was a good paleogeographic back-ground such as large basin, shallow water and gentle slope; the third is that continuous lake transgression and multi-stage slope-break aresignificant conditions for large-sized superposition and merging of sand bodies. A sedimentary model of large-scaled shallow retrogradationfan delta is established, which provides guidance for petroleum prospecting and reserves estimation in Mahu sag.