Coupling Relationship Of Structure-Sedimentation-Diagenesis And Its Effect On Reservoir Quality Controlling In The Huagang Formation Of "X" Gas Field In The Xihu Sag

Tight sandstone gas reservoir is one of the hot spots in unconventional oil and gas research in recent years.In this paper,reservoir characteristics,control factors and mechanism of deep offshore low-permeability tight sandstone gas reservoir in the Xihu Sag were studied.The exploration and development degree of Huagang Formation is low.In previous studies,it is difficult to determine the direction of provenance.The types of sedimentary facies are controversial.The study on the main controlling factors of reservoir densification,such as structure,sedimentation and diagenesis,is weak.As a result,the mechanism of the reservoir in the study area is not well understood and favorable reservoir is hard to predict.Huagang Formation experienced several periods of different types of tectonic movements,which led to great changes in sedimentation and diagenesis.In this context,the combination of structure,sedimentation and diagenesis in different periods is the key to the densification of the reservoir,so it is obviously not enough to analyze the mechanism of the reservoir from a single static factor.In view of the above problems,this paper started with reservoir control factors,such as structural evolution and characteristics,provenance direction,sedimentary facies type,diagenesis characteristics and evolution,to carry out the research on the internal relation,mutual relation and the"coupling"function of the three phases in the key geological historical period,by which the key controlling factors of deep low permeability tight reservoir were determined.On this basis,the"sweet spot"was divided into pore type and fracture type.Based on productivity,the evaluation criteria of"sweet spot"in the study area were determined from two aspects of storage capacity and seepage capacity.Based on the reservoir control factors of structure,sedimentation and diagenesis,structural curvature,favorable sedimentary microfacies and diagenetic were selected to evaluate and predict pore type"sweet spot",and favorable sedimentary microfacies and effective fractures were used to evaluate and predict fracture type"sweet spot".(1)Study on the mechanism of deep low-permeability-dense sandstone reservoirs under the control of three-phase"coupling"of tectonic,sedimentary and diagenetic.1.Sedimentary system under control of tectonic evolution:The tectonic evolution of the southern and central north of the Xihu Sag was studied by the balanced profile technique.Combined with the results of the simulation study of the ancient basin of the the Xihu Sag,the conclusion is as follows:under control of the tectonic evolution,the type of fault basin of the Xihu Sag is“down faulting at the eastern boundary and overlapping at the western boundary”and The sedimentation center and sedimentary center of the Huagang Formation were in the south of the sag,thus forming a sedimentary system of the“Southern Lake,Northern River,Western Delta,and Eastern Fan Delta”.2.Tectonic evolution controlled diagenetic evolution in different areas:After the deposition of the Huagang Formation,due to the change of the subduction direction of the Pacific plate and the Philippine plate,the subsidence center and sedimentary center of the Xihu Sag were offset to the north,making the central and northern parts of the Huagang Formation deeper than the southern part in large diagenetic evolution.The middle and northern parts are in stage A of middle diagenesis,and the southern part is in stage A of middle diagenesis;During the Longjing movement,due to the joint extrusion of the Pacific plate,the Philippine plate and the Okinawa trough,the central uplift developed a large number of echelon anticlines under the strong left-lateral compression and torsion stress.Different anticline elevation lead to different depths and the burial depth determines the difference in the actual compaction of the reservoir.3.The local stress field,provenance and lithofacies of the anticline jointly control the variation of the composition of the Huagang Formation in the study area:Local stress field:The study area is a typical anticline gas reservoir.In different structural parts of the anticline,the difference of tectonic curvature results in the complication of local stress field,thus affecting the degree of compaction.Using the microstructural curvature method to calculate the curvature distribution of the study area,which leads to a conclusion that the curvature of the anticline nucleus is larger than that of the wing,and the curvature of the eastern and northern parts of the nucleus is larger than that of the west and the south.The porosity inversion results of the Huagang Formation are consistent with this pattern.In the drilled position,the X-2 well is in a high curvature position,and the physical properties of this well are significantly better than the other three wells.Provenance:Based on the core data of the X-2 well,the types and contents of cuttings,heavy mineral types and combinations,zircon dating,etc are selected with the microcosmic aspects.Glutenite type,dip angle logging,sandstone inversion,seismic reflection characteristics and other data are selected in macroscopic aspect.Combining the microcosmic and macroscopic data,different layer water system direction and provenance source of H3b,H3c,H4b were proposed.The east source is closer to the northern source,and the rock structure and composition are less mature.At the same particle size,the east source compaction is stronger than the north source.The combination of lithofacies:the paleo-sedimentary environment of the study area is identified through the combination of elemental geochemistry and organic geochemistry.Combined with the observation of sedimentary tectonic features such as phase marks and probability curves of particle size,the disputes over the sedimentary facies types in the study area have been resolved.The Huagang Formation in the study area is an underwater reducing sedimentary environment,and the sedimentary facies of the braided river delta front are developed.The main subfacies in the study area are braided water channels.According to the characteristics of braided waterways,it is further divided into high-energy and low-energy braided waterways.Lithofacies and lithofacic combination types are different in different types of waterways.The coarser grain size have the better physical properties,and the block layering,the parallel layering,the horizontal bedding,and the wavy layer physical properties are deteriorated in turn.Good lithofacies have strong anti-compaction ability,and the primary pores are retained,which promote the dissolution and take away the product of dissolution in time in order to prevent the occurrence of cementation.This case is opposite in the poor lithofacies.The early carbonate-based cementation in the study area mainly occurred at the interface of sand-shale rock(the distance from the sand-shale interface is less than 0.8m),so the sand-shale combination determines the development of early basal carbonate cementation.4.Tectonic movement,sedimentation and diagenesis jointly control the formation and change of fractures in the study area.Through the core,imaging logging,conventional logging,drilling data,logging data and production dynamic data,the macroscopic fractures are comprehensively identified.There are mainly compression-shear fractures in this research area,which are formed mainly by lithology,thickness of stratigraphic single layers,confining pressure and distance from fault during the Longjing movement period.More than 90%of them are filled with iron calcite and more than 60%of which are completely filled.The iron calcite cementation occurs after the densification of the reservoir during the stable subsidence period.The opening and semi-filling fractures are mainly developed in the H4 section.5.The current reservoir of the Huagang Formation is the result of the interaction of the tectonic-sedimentary-diagenetic three-phase in four dimensions.Based on the analysis of the single controlling factors of the above reservoirs,combined with the tectonic evolution history and the diagenetic evolution history,the Huagang Formation reservoir evolution is divided into four stages:the Huagang Formation sedimentary period,the slow subsidence period,the rapid lifting period(including rapid settlement period and rapid uplift period)and stable settling period.The Huagang Formation sedimentary period provides a material basis for the reservoir of the Huagang Formation.The slow subsidence period is the initial stage of reservoir reconstruction,and the sag subsidence center and sedimentation center begin to shift to the north,making the reservoir depth of the Huagang Formation in the north-central part greater than that in southern region.The rapid lifting period is the main reconstruction stage of the Huagang Formation reservoir,and the stable subsidence period is the shaping stage of the Huagang Formation reservoir.These two periods are the key periods for the three-phase“coupling”control of the tectonic-sedimentary-diagenetic rocks.Two important"coupling"control functions have been experienced:1 Rapid settlement period:In this period,the reservoir of Huagang Formation suffered rapid uplift after rapid burial,which was helpful for the preservation of primary pores.They,less faults development,"sandwich"formation development model(mud-sand-mud-sand-mud interbed)of H3 and H4 section,carbonate base-type thin layer cementation zone near the top and bottom interface of sand bodies in H3 and H4 section,strong reservoir heterogeneity,inhibited the flow of fluid and produces abnormal high pressure,which played a role of pore protection for the reservoir of Huagang Formation.In this period,the coupling of structure,sedimentation and diagenesis protected the primary pores.Rapid uplift period.The intense compressive torsion produced a large number of tectonic shear fractures in the target layer.Lithofacies is an important factor controlling fracture density.Fractures are mainly developed in fine sand and medium sandstone lithofacies.Because no large-scale cementation took place in this period,the cracks were in an open state.The coupling of structure,lithofacies and diagenesis controlled the development of fracture zone in this period.2 Stable settling period.The continuous and stable subsidence gradually densified the reservoir of Huagang Formation.The structural curvature is different in different parts of the anticline,and the types of lithofacies and lithofacies assemblages are various,which leads to the obvious difference of reservoir properties in different parts of the anticline.The open structural fractures,as the main dominant migration channel in this period,became the main development site of cementation,and took away the dissolution products of the pores around the fractures,promoting the dissolution of the pores around the fractures.The degree of fracture cementation is different in different positions.Currently,open fractures are mainly distributed in H4 section.The"coupling"of structure,sedimentation and diagenesis in this period contributed to the heterogeneous distribution characteristics of the reservoirs in the present Huagang formation.(?)Evaluation and prediction of“sweet spot”in offshore deep low permeability and tight sandstone gas reservoirs in few wells.The research area has a large area,few wells and lacks production dynamic data.In view of the above problems,this paper used R₃₅ to calculate the theoretical productivity as the evaluation standard for dividing"sweet spot".The distribution of reservoir"sweet spot"in the study area was predicted by combination of seismic data and well data.Specific results are as follows:1.The evaluation criteria of reservoir"sweet spot"was determined.According to the type of reservoir space,the"sweet spot"was divided into pore type"sweet spot"and fracture type"sweet spot".Based on the theoretical productivity calculated by R35,the evaluation criteria of pore type"sweet spot"were divided from two aspects of storage capacity and seepage capacity.The main controlling factors were selected from three aspects of structure,sedimentation and diagenesis,including structural curvature,favorable sedimentary microfacies and diagenetic facies,as the evaluation and prediction parameters of pore type"sweet spot".Sedimentary microfacies and effective fractures were selected as the evaluation and prediction parameters of fracture type"sweet spot".2.The reservoir"sweet spot"of Huagang Formation was predictedBased on the prediction of tectonic curvature,plane distribution of sedimentary microfacies,plane distribution of diagenetic facies and plane distribution of effective fracture zone,the plane distribution of"sweet spot"was obtained by overlapping the evaluation parameters.1 Prediction of pore type"sweet spot":The microstructure curvature method was used to calculate the plane distribution of the structural curvature in the study area.And the seismic sedimentology method was used to predict the plane distribution of the sedimentary microfacies.The apparent compaction rate and apparent dissolution rate were selected as the evaluation parameters of diagenetic facies,and the plane distribution of seismic diagenetic facies was predicted by well seismic combination.On this basis,the plane distribution of pore type"sweet spot"is obtained by superposition of different small layer structures with high curvature area,favorable sedimentary microfacies,favorable diagenetic facies and gas-bearing range.The pore type"sweet spot"is mainly distributed in the middle of H3b-2and H3b-3 small layer and the south of H3c small layer.2 The fracture type"sweet spot"prediction:By selecting sensitive rock physical parameters and using bayesian discrimination,effective fracture plane distribution was obtained by Matlab calculation.The effective fracture plane distribution,favorable sedimentary microfacies plane distribution and gas-bearing range facies were superimposed to obtain the plane distribution of fracture type"sweet spot".The results showed that the fractured sweet spot was mainly distributed in the H4 segment,the upper part of the H4section was mainly distributed in the southwest and north-central part of the study area,and the lower part of the H4 section was only distributed in the small range in the south-central part of the study area.