Sedimentary Characteristics And Genetic Types Of Eogene Daxing Conglomerate In The Langgu Depression Of Bohai Bay Basin, East China

Daxing Conglomerate is located on the downthrown side of Daxing Fault in the westernLanggu Depression of Bohai Bay Basin. It is contacted or surrounded by a big set of maturehydrocarbon source rocks and has the huge resource potential. In this paper, the geologicalbackground, genetic types and sedimentary characteristics of the Daxing conglomerate in thethird member of the Oligocene Shahejie Formation (Es3) in the Langgu Depression werestudied in detail based on seismic profiler, logging, cores, petrographic thin-sections andscanning electron microscope (SEM) photos. Daxing Conglomerate was mainly developed inthe intensively faulting period of Langgu Depression. The structure evolution and faultactivity controlled the formation and distribution of Daxing Conglomerate. The pre-Paleogenetectonic movement controlled the source types of conglomerate, the Paleogene tectonicmovement controlled the vertical distribution of conglomerate, and the episodic activity andmigration controlled the genetic types and evolution. The stratum on the Daxing Bulge weremainly dominated by dolomites in Jixian System, Cambrian and Ordovician limestones andlocal Mesozoic clastic rocks before the deposition of the lower submember of the Es3. Thestratum on the Daxing Bulge were mainly characterized by dolomites in Jixian System,Cambrian limestones and local Ordovician limestones after the deposition of the lowersubmember of the Es3. The stratum on the Daxing Bulge were mainly dominated bydolomites in Jixian System and local Cambrian limestones after the deposition of the middlesubmember of the Es3. Under the control of geological background of Daxing Bulge, the rocktypes of Daxing Conglomerate were mainly matrix-supported conglomerates andgrain-supported conglomerates. The gravels in conglomerates were dominated by dolomitesand limestones. The sedimentary structures in conglomerates were simplex, which weredominated by massive bedding and contemporaneous deformation structure. Based on thedepositional setting, three genetic models of conglomerate, including gravity flow channel,grain flow nearshore subaqueous fan and mud flow nearshore subaqueous fan, have been setup depending on cores, seismic facies and electrofacies. The gravity flow channel wascharacterized by matrix-supported conglomerates and grain-supported conglomerates. Itshowed the positive sequence overlying by main channel and subchannel from bottom to topin vertical sequence. In logging caves, main channel was high amplitude box shape, and subchannel was in jugged box shape or bell shape in Sp and Rt caves. In seismic section, thegravity flow channel showed as wedgy filling with lateral accretion or nearly horizontal onlapstructure in cross section and obvious progradational reflection configuration on longitudinalsection. The grain flow nearshore subaqueous fan was dominated by grain-supportedconglomerates. In the vertical section, it was successively deposited by inner fan, middle fanand outer fan from bottom to top, showing the upward-fining retrogradation sequence. Inlogging caves, it showed as box shape or jugged box shape with characteristics of low Grvalue and high Rt value. In seismic section, it was the strong amplitude wedge set extendingto the centre of basin. The mud flow nearshore subaqueous fan was characterized bymatix-supported conglomerates. In the vertical sequence, it successively deposited by innerfan, middle fan and outer fan from bottom to top, showing the upward-fining retrogradationsequence like the grain flow nearshore subaqueous fan. In Sp and Rt caves, it showed asjugged bell or box shape. In seismic section, it showed as strong amplitude chaotic seismicreflection configuration with the border fingerlikly contacting with lacustrine mudstonereflection. Under the control of gravels, structure, genetic types and facies tract, theconglomerate bodies were grain flow nearshore subaqueous fan, gravity flow channel andmud flow nearshore subaqueous fan in poroperm characteristics and hydrocarbon productionfrom good to bad. Therefore, the establishment of three genetic models of conglomerate hasan important significance for fine exploration of oil and gas of Daxing Conglomerate inLanggu Depression.