冲积扇“颗粒支撑砾岩”的成因和分布及其油气地质意义

Genetic mechanism,distribution and significance for hydrocarbon exploration of the grain-supported conglomerate in alluvial fans

为明确冲积扇储层成因及分布规律,结合油气勘探实践及野外露头观察,总结归纳了不同机制下冲积扇颗粒支撑砾岩的类型及其沉积分布特征.本文认为冲积扇洪水期、间洪期及碎屑(泥石)流期发育不同类型的颗粒支撑砾岩,扇中的片流型、辫/曲流带型是油气聚集的甜点区.研究结果表明:1)冲积扇中的颗粒支撑砾岩物性好,杂基质量分数小于15%,其包含分选磨圆很差的角砾岩、磨圆分选很好的"同级颗粒支撑"砾岩和磨圆很好但分选较差的"多级颗粒支撑"砾岩;2)基于冲积扇的形成机制及沉积过程,将颗粒支撑砾岩划分为6种成因类型:岩石流型、碎屑(泥石)流型、片流型、辫/曲流型、二次改造流体型及风成型;3)6种类型分布规律具有差异:岩石流型以不规则状在山前堆积、碎屑(泥石)流型以堤坝状分布在舌状体前端及边部、片流型层状分布,辫/曲流型以条带状分布在沟槽底部、侧缘槽、心滩、河道侧缘坝体等部位,二次改造型以透镜体分布,风成型分散堆积.综合认为冲积扇颗粒支撑砾岩的分布规律在空间上具有关联性,片流型、辫/曲流型具有较大的勘探开发潜力,大型砂砾岩油田中颗粒支撑砾岩储层孔隙度高达18%,是油气的重要储层,也是影响开发效果的窜流通道.

To ascertain genetic mechanism and distribution of high-quality reservoirs in alluvial fans,the type characteristics and distribution of grain-supported conglomerate with different genetic mechanism are summarized with the combination of hydrocarbon exploration and outcrop observation.It is indicated that different lithofacies of grain-supported conglomerate in alluvial fans are developed in flooding periods,inter diluvial periods and debris flow periods.Furthermore,lithofacies of sheet flood and fluvial dominated grain-supported conglomerate in mid-fan are sweet points.The results show that:1)Grain-supported conglomerate in alluvial fans,including poor sorting and bad rounded breccia,good sorting and rounded gravel with same granular supported clast,and poor sorting and good rounded gravel with different granular supported clast,is characterized by less than 15%of matrix mass fraction.2)In the basis of genetic mechanism and deposition process,grain-supported conglomerate can be divided into 6 types of lithofacies including:slump type,debris flow type,sheet flood type,fluvial dominated type,secondary fluid processes type,and aeolian type.3)There are differences among these 6 types of lithofacies.The slump type is irregular and accumulates in front of the mountain;the debris flow type occurs in the front and side of the tongue shaped body with dam shape;the sheet flood type forms with level layer distribution;the fluvial dominated type develops at the bottom of river,one side of grooves,diara and beach with banded;secondary fluid processes type forms with lenticular shape;and aeolian type is characterized by scattered accumulation.The comprehensive evaluation and analysis suggest that the distribution of grain-supported conglomerate in alluvial fans is related in space.The sheet flood type and fluvial dominated type have great exploration potential.Grain-supported conglomerate reservoir in large-scale glutenite oil field is an important reservoir of oil and gas with the porosity of 18%,and is also a cross flow channel affecting the development effect.