西湖凹陷Z地区平湖组煤系地层沉积背景及水动力特征

Depositional setting and hydrodynamic characteristics of coal measure strata in Pinghu Formation,Z area,Xihu sag

西湖凹陷是中国近海最重要的含油气凹陷之一,在凹陷断陷期末期阶段发育一套巨厚煤系地层,是西湖凹陷主力烃源岩层系,由于受到多解性的影响,很难通过岩性组合、沉积构造等宏观特征详细解释研究区沉积背景及水动力特征,尝试通过煤系地层干酪根显微组分,煤系地层显微岩石学特征等对西湖凹陷Z地区平湖组煤系地层沉积环境及水动力特征展开分析。研究表明,研究区不同地区干酪根显微组分存在较大差异,岩心具有交错纹层状、条带状、水平纹层状、结核状4种形态特征的菱铁矿,均指示Z地区平面及纵向上煤系地层沉积背景及水动力特征存在较大的差异:低凸A区沉积时期以弱氧化-还原沉积环境为主,水浅且间歇性出露水面,其上可发育天然堤与洼地,有利于水道发育;凹陷B区是受外缘低凸起遮挡而形成的半封闭潟湖,整体处于还原沉积背景,在大潮期间,潮流可越过低凸起,将隆起区及隆起区外侧碎屑物搬运至凹陷区而形成冲越扇;低凸C区长期位于水面之上,处于氧化环境,平中段时期受到间歇性潮汐的影响而被水体覆盖,从而形成浅水沼泽;凹陷D区为水体相对较深的沉积背景,是三角洲前缘沉积的有利场所。总体而言,Z地区平湖组沉积环境及水动力特征自南向北受地貌影响存在较大差异:低凸A及C区因长期暴露地表,整体表现为氧化环境,河控为主,间歇性受到潮流影响;凹陷B及D区,整体处于还原沉积环境,受潮汐影响较大。通过对西湖凹陷Z地区沉积背景及水动力特征认识的深化,为该地区勘探部署提供了重要理论支撑。

Xihu sag is one of the most important hydrocarbon bearing sags nearshore China.A set of extremely thick coal measure strata is developed at the end of the fault depression period of the sag,it is the main source rock series of Xihu sag.Due to the influence of multiple solutions,it is difficult to explain the depositional setting and hydrodynamic characteristics of the study area in detail through the macrofeatures such as lithological combination and sedimentary structure.This paper attempts to analyze the sedimentary environment and hydrodynamic characteristics of the Pinghu Formation coal measure strata in Z area of Xihu sag through kerogen macerals and micropetrological characteristics of coal measure strata.The study shows that there are great differences in the kerogen macerals of different regions in Z area,and the core exhibits four morphologic characteristics:cross laminated,banded,horizontally-laminated and nodular siderite.Both indicate that there are great differences in the depositional setting and hydrodynamic characteristics of coal measure strata in the plane and vertical directions of Z area.The depositional stage of A region in low uplift is dominated by weak oxidation-reduction sedimentary environment with the shallow water and intermittent exposure to the water surface,and natural levee and bottom land can be developed on it,which is conducive to the development of water channel.B region of the sag is a semi-enclosed lagoon formed by the barrier of the low uplift on the outer edge,and the whole is in a reducing depositional setting.During the spring tide,the tidal current can pass over the low uplift and transport the debris from the upwarped district and outside the upwarped district to the depressed area to form a washover fan.C region of the low uplift has been located above the water surface for a long time and is in an oxidizing environment.It is covered by water body under the influence of intermittent tides in middle Pinghu period,thus forming a shallow swamp.D region of the sag is a depositional setting that the water body is relatively deep and a favorable place for delta front deposition.In general,the sedimentary environment and hydrodynamic characteristics of Pinghu Formation in Z area differ greatly from south to north due to geomorphic impacts.The A and C regions of low uplift,due to long-term exposure to the surface,their overall performance is oxidized.The environment is dominated by rivers and intermittently affected by tidal current.The B and D regions of the sag are overall in a reducing sedimentary environment and are greatly affected by tides.The deepening understanding of the depositional setting and hydrodynamic characteristics in the Z area of the Xihu sag provides important theoretical support for the exploration and deployment in this area.