| With the continuous increase of global oil and gas demand and the continuous development of exploration technology,oil and gas exploration has gradually shifted from shallow to deep.Bedrock oil-gas reservoirs represented by magmatic rocks and metamorphic rocks are paid more and more attention.At present,hundreds of industrial bedrock oil-gas reservoirs have been discovered in more than 30 basins in the world,showing great exploration potential of bedrock oil-gas reservoirs.Bedrock oil-gas reservoirs in China are mainly distributed in Bohai Bay Basin,Songliao Basin,Liaohe Basin,Donghai Basin in the east,Pearl River Mouth Basin,Qiongdongnan Basin,Beibuwan Basin and Qaidam Basin in the west,and most of them belong to Paleozoic and Mesozoic buried hill oil-gas reservoirs,while the research and exploration degree of Archean crystalline bedrock oil-gas reservoirs is relatively low,and the discovery degree of resources only accounts for 11%.In addition,the understandings of the Archean crystalline bedrock weathering crust still have some deficiencies,which are manifested in that the formation and evolution mechanism and distribution law of crystalline bedrock weathering crust are relatively weak,the classification of effective reservoirs and the distribution of high-quality reservoirs are not clear,and the hydrocarbon accumulation law and favorable exploration zones have not been clarified.These shortcomings restrict the exploration of bedrock weathering crust oil-gas reservoirs.To solve these problems,based on the outcrop,core,logging,imaging logging and seismic data,this paper takes the weathering crust of Archean crystalline bedrock in Jiyang Depression as the research object,and the Chenjiazhuang uplift and Binxian uplift in the northern belt of Dongying Sag,and Chengdao-Zhuanghai area in the northeast of Jiyang Depression are dissected.the formation and distribution of the weathering crust of crystalline bedrock and the law of oil and gas accumulation are studied.Four main understandings in this paper has been obtained:(1)A cyclic evolution model of crystalline bedrock weathering crust in Jiyang Depression has been established,and a new scheme of the "1-2-2-1" type longitudinal evolution structure division of weathering crust has been proposed(1 indicates that the bedrock weathering crust is a single-layer structure,2 indicates that the bedrock weathering crust is a double-layer structure).It is pointed out that the longitudinal structure of the weathering crust exhibits different characteristics in different stages of evolution.In the initial stage of the formation of weathering crust,magma invades into the surface and cools into rock.After weathering and denudation,a "single layer structure" of the disintegration layer is formed;in the early stage of weathering crust evolution,the top of the disintegration layer begins to develop into a dissolution layer,resulting in a "double layer structure" of dissolution layer and disintegration layer;in the middle stage of weathering crust evolution,the dissolution layer evolves into an eluvium,forming a "double layer structure" of eluvium and disintegration layer;in the late stage of the weathering crust evolution,the eluvium at the top is completely weathered and denudated,and only the "single layer structure" of the disintegration layer is retained,which constitutes a weathering cycle in which the weathering crust changed from single-layer structure in the initial stage to double-layer structure in the early and middle stages and then to single-layer structure in the late stage.(2)The various transformation and destruction effects of crystalline bedrock weathering crust are clarified,a new mechanism for the preservation of weathering crust is proposed,and a transverse differential distribution model of the dynamic genesis of crystalline bedrock weathering crust is established.It is believed that bedrock weathering crust has undergone various transformation and destruction processes during its formation,such as structural tilting,fracture destruction,surface fluvial abrasion,and wind and atmospheric water weathering.It is pointed out that the covering effect of the later strata is crucial for the preservation of the bedrock weathering crust and the overlying strata in the lower parts of the gentle slope area are covered earlier,which is more advantageous for the preservation of the bedrock weathering crust;It is believed that there is a huge difference in the types of weathering crust on different elevations,different slopes and different structural belts.Only disintegration layers are developed at the top of the structure,eluvium are developed in the marginal areas of the high structural part,and dissolution layers are developed in middle-low structural parts.The disintegration layer is widely distributed.(3)A new scheme for effective reservoir classification of bedrock weathering crust in Jiyang Depression has been proposed,and a high-quality reservoir distribution prediction method combining geology and seismic is formed.It is pointed out that the effective reservoirs of crystalline bedrock weathering crust can be divided into three types based on the degree of evolution,types of reservoir space combination,porosity and permeability characteristics,oil-bearing properties,and preservation conditions from good to bad,namely,the dissolution layer is a type Ⅰ high yield oil reservoir,the eluvium is a type II low yield oil reservoir and the disintegration layer is a type III oil bearing reservoir.It is believed that the development zone of the dissolution layer in the middle and low parts of the structure and deep buried low bulge parts is a high-quality reservoir distribution area.It is pointed out that the medium frequency,low continuous,and medium amplitude seismic facies are typical seismic facies types of high-quality reservoirs,and a set of high-quality reservoir interpretation process of bedrock weathering crust is formed,which takes frequency division processing of seismic data,seismic facies analysis and seismic sensitive attribute interpretation as the core and combines well seismic calibration.(4)Four different hydrocarbon accumulation models of crystalline bedrock weathering crust in Jiyang Depression are established,and two favorable exploration zones for bedrock oil-gas reservoirs are defined.Specifically,the hydrocarbon accumulation models of the weathering crust of Archean crystalline bedrock in Jiyang Depression can be divided into four types,namely,hydrocarbon accumulation model in the high part of the uplift has the characteristics of near-source lateral bidirectional hydrocarbon supply-fault and unconformity transport-late stratigraphic cover preservation-poor reservoir conditions and preservation conditions,hydrocarbon accumulation model in the gentle slopes near high structural parts is characterized by near-source lateral unidirectional hydrocarbon supply-unconformity surface and fault transport-late stratigraphic cover preservation,hydrocarbon accumulation model in the middle and low parts of the structure is characterized by source-crust connectivity,unconformity surface transport and paleo-uplift reservoir-controlling,hydrocarbon accumulation model in the deep buried low bulge is characterized by near-source lateral bidirectional hydrocarbon supply-fault and unconformity surface transport-paleo-uplift reservoir-controlling.It is believed that the middle and low parts of the structure are favorable exploration areas for bedrock oil-gas reservoirs in the northern area of Dongying Sag,and the deep buried low bulge parts are favorable exploration areas for crystalline bedrock weathering crust oil-gas reservoirs in the Chengdao-Zhuanghai area.The four research results obtained in the paper have important theoretical and applicable value for determining the geological origin,reservoir distribution,and oil and gas exploration of crystalline bedrock weathering crust in other rift basins worldwide. |
