Main Controlling Factors Of Hydrocarbon Accumulation Of Carboniferous Volcanic Rocks In Eastern Fukang Depression

The formation and distribution of the Carboniferous volcanic rock oil reservoirs in the eastern Fukang depression show inconspicuous regularities,and hydrocarbon accumulation rules of them are very complex.To clarify hydrocarbon accumulation rules of Carboniferous volcanic rock oil reservoirs in the study area,with the consideration of tectonic and sedimentary evolution characteristics,based on seismic,well logging,geological logging,drilling and testing data,typical reservoir dissecting and analysis of hydrocarbon accumulation elements were conducted.The hydrocarbon enrichment regularities were summarized,the main controlling factors of hydrocarbon accumulation were clarified and the hydrocarbon accumulation patterns were established.The results of typical reservoir dissecting showed that physical property characteristics of Carboniferous crude oil from different blocks existed some similarities and differences.The crude oil from different blocks had similar characteristics of wax content and freezing point,which showed medium-high and medium values respectively.The crude oil from Wellblock Bei-32 was medium-density and heavy oil with relatively high density and viscosity values,while the crude oil from Block xiquan and Wellblock Fu-10 were normal oil.The crude oil from different blocks had high saturated hydrocarbon content,especially the content was over 90% in Well Fu-10.A variety of Carboniferous reservoir types were developed in the study area.Shaqiu-6 reservoir and Fu-10 reservoir were fault block type.Reservoirs of Xiquan oilfield were fault block or fault nose and stratigraphic style that were laterally sealed by unconformity.Anticline reservoir was developed in Wellblock Bei-32,which was controlled by the shape of anticline.Three types of source rocks existed in Carboniferous-Permian strata.Hydrocarbon generation potential of the dark shale of Pingdiquan Formation in the middle Permian was best,the tuffaceous source rock of Bashan Formation in upper Carboniferous showed general level,while the dark shale of Bashan Formation was bad.The results of oil-source correlations indicated that the Carboniferous cruel oil of Block Xiquan derived from tuffaceous source rocks of Bashan Formation,and the origin of cruel oil of Block Shaqiu and Wellblock Fu-10 was the dark shale of Pingdiquan Formation.The major reservoir lithology of Carboniferous strata was andesite,basalt,tuff,breccia,and tuffaceous sandstone in the study area.Intercrystalline dissolution pores,matrix dissolution pores,structural fractures and dissolution fractures were main reservoir spaces.The permeability of the upper weathered crust reservoir was better than that in the lower part of the Carboniferous reservoir,and the permeability of the volcanic reservoir in Block Xiquan was better than that in other blocks.There were three types of reservoir-cap assemblages in the study area,which were an important factor for hydrocarbon to accumulate.In addition,faults were the main pathway for hydrocarbon migration in the study area,whose role was cutting apart the source rocks and migrating hydrocarbon vertically.Vertically,the Carboniferous oil in the study area was mainly accumulated in the weathered crust.Horizontally,the distribution range of the Carboniferous oil in Block Xiquan was mainly controlled by thicknesses of Carboniferous tuffaceous source rock and cap rock.The thickness of dark shale at the bottom of Pingdiquan Formation dominated the distribution of oil in Block Shaqiu and Wellblock Fu-10.The hydrocarbon accumulation pattern of Block Xiquan is summarized as “tuffaceous source rock is oil source,faults migrating hydrocarbon vertically,and hydrocarbon accumulating in the weathered crust”.It is summarized as “dark shale of Pingdiquan Formation is oil source,faults migrating hydrocarbon(this part may not exist),and hydrocarbon accumulating in the weathered crust” in the Block Shaqiu and Wellblock Fu-10.