Upper Paleozoic Source-reservoir Configuration Evolution And Gas-water Distribution Response In Northern Ordos Basin

The configuration relationship between source rocks and reservoirs(source reservoir configuration)is the main controlling factor for the formation and enrichment of tight sandstone gas reservoirs in large craton basins.In the transition zone(basin margin transition zone)between the large hydrocarbon generation center of the basin and the structural uplift area around the basin,affected by multi-stage tectonic movement,the source reservoir allocation and its evolution process are more complex and the plane difference is greater than that inside the basin.Under the control of source reservoir allocation relationship,the gas reservoir type and gas water distribution in the basin margin transition zone are different from both the continuous gas reservoir distributed in a large area in the basin and the discontinuous structural gas reservoir in the uplift area of the basin margin.The reservoir forming model on the continuous discontinuous transition zone has not been established.Therefore,the research on the evolution of source reservoir allocation on natural gas accumulation and gas water distribution is of great significance to improve the oil and gas distribution model of craton basin.The exploration breakthrough of Dongsheng gas field in Hangjinqi area in the north of the Ordos Basin has realized the expansion of the exploration activities of tight sandstone gas reservoirs from basin center to basin margin.By the end of 2021,the Upper Paleozoic reservoirs in Hangjinqi area in the basin margin transition zone of the northern Ordos Basin has a total proved natural gas reserves of nearly 200 billion cubic meters,which has become one of the most important gas-producing fields in China.It is confirmed that the basin margin transition zone has the potential and geological conditions to form medium to large-sized tight sandstone gas fields.The Permian tight sandstone gas reservoir in the Hangjinqi area can be used as a model for the study of tight gas reservoirs with continuous-discontinuous accumulation in basin margin transition zone in large-scale craton basin.Based on new exploration and development dataset in Hangjinqi area,this study integrated structure-sedimentary evolution history in basin margin transition zone,and geological-seismic data in order to analyze the effect of structure and sedimentary evolution history on source-reservoir distribution patterns.This study also investigated the evolution process of natural gas-filling driving force and resistant force.Based on the gas-water distribution patterns and occurrence forms,this study summarized the characteristics and formation modes of differential source-reservoir configuration,the evolutionary dynamic mechanisms of source rock,reservoir,and gas accumulation,as well as gas-water distribution modes.The effect of high-quality source rock distribution,reservoir heterogeneity and source-reservoir driving-resistant force on gas-water distribution were documented.Combined with the analysis of natural gas migration and accumulation,this study summarized the spatial-temporal allocation of source-reservoir and the response mechanism of gas-water distribution in the basin margin transition zone,and established the natural gas accumulations models by zones.This study also summarized the accumulation mechanism and gas water distribution law of large and medium-sized tight lithologic gas reservoirs under the background of large area and low abundance in the basin margin transition zone,and established the natural gas accumulation model with the characteristics of the basin margin transition zone,so as to provide reference for the exploration and development of the same type of oil and gas reservoirs in the basin margin transition zone in large craton basin,and improve the exploration and development efficiency of tight gas in the basin margin transition zone.At the same time,this study enriches the theory of low-permeability tight oil and gas reservoirs with continuous-discontinuous accumulation in the margin transition zone of craton basin.The main results are as follows:1.The middle-upper Proterozoic and Paleozoic geological structures in the northern margin of Ordos Basin can be divided into four overlapping types: fault depression-slope,fault depression-uplift,uplift-fault terrace and sag-slope.The tectonic superposition evolution in the north of Ordos basin controls the spatial-temporal relationships between the Carboniferous Taiyuan-Permian Shanxi coal-bearing source rocks and the Permian Lower Shihezi sandstone reservoirs.Controlled by paleogeographic positions that is characterized by high position in northwest and low position in southeast,the coals are mainly distributed in the Shiligahan sag-slope and Duguijiahan fault depression-slope superposition area.Coals are locally distributed in the Shiguhao uplift-fault terrace in the northeast.The difference of burial history between the east and west of the study area results in the difference of the distribution of coal seam maturity,which controls the high distribution in the west and low distribution in the east of the Carboniferous-Permian source rock maturity.Influenced by the paleogeomorphic positions,the Taiyuan-Lower Shihezi Formations in Hangjinqi area has the source-sink system and sand body distribution patterns that are characterized by East-West zoning,North-South zoning and vertical differentiation.The difference of maximum burial depth between the east and the west of Permian caused the higher densification degree of sandstone in He 1 member in the west and lower in the east.2.The reservoir lithology of Lower Shihezi Formation,Shanxi Formation and Taiyuan Formation is mainly lithic sandstone,feldspathic lithic sandstone,lithic quartz sandstone and quartz sandstone.The particle size is mainly coarse sandstone,followed by gravelly coarse sandstone and glutenite.The particle sorting is medium to good,and the particle roundness is mainly sub-angular.The clastic particles are mainly in point and line contact.The cementation is mainly pore type,and the cement composition is mainly siliceous cement.The upper Paleozoic reservoir is typically a tight reservoir with ultra-low permeability.From the northeast to the southwest of the study area,the quartz content of the Upper Paleozoic sandstones in Hangjinqi area gradually increases,while the grain size gradually decreases,and the reservoir physical properties varies from low permeability sands(average porosity 15%,permeability 2.0m D)to tight sands(average porosity 8%,permeability 0.5m D).Multiple diagenetic processes occurred in the Upper Paleozoic reservoirs in Hangjinqi area,and mineral dissolution is the main constructive diagenetic process,whereas compaction and diverse cementation are the main destructive diagenetic process.According to the occurrence and distribution characteristics of diagenetic inclusions,three stages of diagenetic inclusions can be identified.The paragenetic sequence of the Lower Shihezi reservoirs in Hangjinqi area is as follows: sedimentation—compaction—clay coating—first generation of quartz overgrowth—early calcite cementation—acid dissolution—structural fracture—second generation of quartz overgrowth—hydrocarbon charging—ankerite cementation—calcite cementation—quartz overgrowth.3.The source rocks in Hangjinqi area are coals,carbonaceous mudstones and dark mudstones of the Taiyuan and Shanxi Formations.In the Taiyuan and Shanxi Formations in Hangjingqi area,vitrinite is the dominant maceral type,accounting for80% in the Taiyuan coals,which is higher than that of Shanxi coals.The vitrinite reflectance of coal samples ranges from 1.04% to 1.85%,with an average of 1.34%.The Upper Paleozoic source rocks in the south of the Borjiang Haizi fault are currently in high mature stage,whereas source rocks in the Shiguhao area are still in the early mature stage.The simulation of hydrocarbon generation history shows that most of coaly source rocks of Taiyuan and Shanxi Formations in Hangjinqi area reached hydrocarbon generation peak at the end of Early Cretaceous(～ 115-100Ma),indicative of the beginning of massive oil and gas migration,charging and accumulation in the study area.The gas generation rate of source rocks is high in the west and low in the east,the highest in Xinzhao area and the lowest in Shiguhao area.4.Homogenization temperature measurement of reservoir inclusions combined with laser Raman spectroscopy shows that the natural gas of Heyi Member in Hangjinqi area has three charging periods,occurred at the stage of 190-140 Ma and 110-100 Ma during deep burial,and the stage of 80 Ma during uplift,and the charging stage of110-100 Ma at the end of Early Cretaceous is the key stage of natural gas charging and accumulation in the study area.The densification and near densification of sandstone reservoirs in the Xinzhao,Duguijiahan and Shiligahan areas mainly occurred before110 Ma,and can be summarized as first densification and then gas accumulation.The reservoir in Shiguhao area has not been densified,which is a type reservoir of gas accumulation without densification.The critical time of gas accumulation in the Xinzhao area is high charging power zone,and the Duguijiahan area is medium-high charging power zone,which has the dynamic conditions of natural gas enrichment;whereas,the Shiligahan area is medium charging power zone,in which the driving force for natural gas charging in insufficient in the north and sufficient in the south.Due to the lack of high mature source rocks,the Shiguhao area is characterized by low excess pressure.On the other hand,the Shiguhao area is located in the regional structural high for a long time,which is favorable for gas migration from the south.5.There are six types of gas and water occurrence state in the Hangjinqi area.High gas-saturation reservoir and weak differentiation gas reservoir developed under the background of strong charging power is the dominant type.Under strong dynamic background,the gas-water overlapping layer and gas-water two-phase mixed layer are mainly developed,and the high saturated gas layer is locally developed.Under the background of weak charging power,gas-water two-phase mixed layer and weakly differentiated gas bearing layer are mainly developed.Isolated water layers are developed in different traps.The main factors controlling gas and water distribution in different parts of the continuous-discontinuous gas accumulation transition zone are defined.In the slope area close to source rocks,gas and water distribution is mainly controlled by source-reservoir configuration,the upward blocking condition of river channels,and the relationship between source-reservoir driving and resistant forces.In the uplift area,gas and water distribution is mainly controlled by the superposition relationship between dominant sand bodies and structural traps.6.According to the temporal and spatial configuration of the upper Paleozoic source and reservoir and the evolution process of dynamic accumulation in Hangjinqi area,it can be divided into three types.In the western gentle slope area(Xinzhao and duguijiahan area),the high charging force matches the high reservoir resistance in the accumulation stage,and the natural gas is prefered to be charged,without gas-water differentiation,and the buoyancy is limited and without fluid adjustment in the uplift stage.In the middle transition zone(Shilijiahan area),during the accumulation period,the middle charging force matches the middle-high reservoir resistance,the gas is prefered to charge,and the local gas-water differentiation or weak differentiation occurs.During the uplift,buoyancy plays a role in the sandstone with good physical properties,and the gas-water adjustment occurs locally.In the eastern uplift area(Shiguhao area),under the action of buoyancy,the gas/water differentiation is obvious,and the gas/water adjustment occurs with the structural changes during the uplift period.7.From the uplift of basin edge(discontinuous accumulation)area to the slope area inside(continuous accumulation),with the increase of burial depth,the source rock maturity gradually increases,the sandstone/formation ratio gradually decreases,and the reservoir porosity gradually decreases,the source-reservoir pressure difference increases from 0 to 30 Mpa,and the median reservoir pressure transitions from 10 Mpa to 28 Mpa in the key period of reservoir formation.The proportion of free water in reservoir decreases gradually,the proportion of bound water increases gradually,and the proportion of natural gas increases first and then decreases.The dynamic threshold of gas enrichment is when charging power is greater than reservoir resistance of 7Mpa,and the geological threshold of buoyancy is when reservoir/ground ratio is greater than 0.3and the permeability of reservoir is greater than 1.0 m D.8.According to the differences among petroleum systems and trap types,the Hangjinqi area can be divided into four hydrocarbon accumulation zones,including source tight—lithologic reservoirs,source tight stratum—lithologic reservoirs,source low-permeability—structural reservoirs,and low-permeability structural—lithologic reservoir.According to the spatial-temporal configuration of source-reservoir,the process of natural gas filling,accumulation and adjustment,and gas and water distribution characteristics,this study established the "reservoir densification and large-area low-permeability lithologic gas reservoir " in slope area within source rocks,and the "gas accumulation with non dense reservoir and effective trap enrichment mode" in the uplift area beside source rocks.