Characteristics And Main Controlling Factors Of Diabase Reservoirs In Huimin Sag,Bohai Bay Basin

Diabase is widely intruded into the thick mudstone of the third member of Shahejie Formation in Huimin Sag,Bohai Bay Basin.The diabase reservoir in this area has strong heterogeneity.Currently,there is a lack of in-depth understanding of the main controlling factors for high-quality diabase reservoirs in this area.Taking the diabase development area in Linnan Subsag of Huimin Sag as a typical example,this paper comprehensively utilizes research methods such as core observation,casting thin section,porosity analysis,scanning electron microscope observation,and rock physical property testing,combined with relevant logging data,to conduct in-depth research on the diabase reservoirs in the area,and clarify the lithology and lithofacies characteristics,invasion stages,reservoir space types,and microscopic pore throat characteristics of the diabase in the area,To explore the control factors of favorable reservoirs in order to provide reference significance for further exploration and development of diabase reservoirs in Huimin Sag,Bohai Bay Basin,the specific understanding is as follows:1.Lithology and lithofacies characteristics:Based on the core description and microscopic thin section characteristics,the diabase in the third member of the Shahejie Formation in Huimin Sag is further divided into three sub categories:coarse grained diabase,medium grained diabase,and fine grained diabase,using 0.5 mm and 1 mm interval boundaries,based on the statistical results of the measurement of the long axis length of plagioclase in the diabase,and a lithological identification chart is established;Establish identification criteria for diabase intrusive facies subfacies in the third member of Shahejie Formation in Huimin Sag and divide the intrusive subfacies into marginal subfacies,transitional subfacies,and central subfacies;Taking the diabase from the third member of Shahejie Formation in Well Shang 742 as the research object,a petrographic analysis was conducted on it.Based on this analysis,the large-scale magmatic intrusion stages of the diabase from the third member of Shahejie Formation in Huimin Sag were analyzed.It is believed that the diabase feldspar grain size in the third member of Shahejie Formation in Huimin Sag exhibits a longitudinal inversion and a sudden change in the petrographic boundary on the core,indicating that this was not formed by a stage of magmatic intrusion.Based on this,the diabase of the third member of the Shahejie Formation in Huimin Sag is divided into two stages of magmatic activity according to the lithology of the surrounding rock contacted by the diabase.2.Types and characteristics of reservoir space:The diabase reservoir space of the third member of the Shahejie Formation in Huimin Sag is divided into primary and secondary types,wherein,the primary reservoir space includes:pores,intercrystalline pores,condensation contraction fractures,and cleavage fractures;The secondary reservoir space includes:corrosion pores（including intracrystalline corrosion pores,matrix corrosion pores）,clay mineral intercrystalline pores,corrosion fractures,structural fractures,etc.The study area is mainly composed of secondary pores and fractures.Specifically,intracrystalline and matrix corrosion pores are the most common reservoir spaces,and primary pores and fractures are partially or completely filled with secondary minerals such as calcite and saponite.The porosity of the diabase reservoir in the third member of the Shahejie Formation in Huimin Sag is mainly between 0.3%and 9.5%,with an average of 4.2%;Permeability is mainly between 0.025 and 0.792×10^-3μm²,average 0.072×10^-3μm² is a typical medium low porosity and ultra low permeability tight reservoir.Based on the characteristics of capillary pressure curve,pore size distribution range,and physical properties,the classification and evaluation criteria for diabase reservoirs in Huimin Sag were established.Among them,the microscopic pore throat characteristics of Class I superior reservoirs,Class II better reservoirs,and Class III medium poor reservoirs gradually transform from coarse to medium large pore throat to fine to micro pore throat.Based on this,the effects of pore structure characteristic parameters on porosity and permeability were studied respectively.Among them,porosity is closely related to the median pore throat radius,and permeability is the most sensitive to the maximum connected pore throat radius,The results of gas porosity and permeability tests and high-pressure mercury injection tests show that coarse grained diabase is a relatively high-quality reservoir.3.Main control factors of reservoir:Research shows that the main control factors of high-quality diabase reservoirs in the third member of the Shahejie Formation in Huimin Sag include lithology,lithofacies,dissolution,and tectonic activity.Lithology and lithofacies are the basis for reservoir development,with favorable lithology being coarse grained diabase and favorable lithofacies being the central subfacies;By dividing the diagenetic stages of diabase in the study area and analyzing the characteristics of diagenesis,diagenesis can be divided into two categories:constructive diagenesis and destructive diagenesis,with the strongest effect on the construction of high-quality reservoirs being dissolution;Tectonic activity is the decisive factor for the development of diabase reservoirs.The early diabase intrusions were emplaced into the sedimentary layer under tectonic action,forming a reservoir space.Later intrusions subsequently intruded into the fracture space generated by the structure in the early intrusions,and due to the small temperature difference with the surrounding rock,high-quality reservoirs such as coarse grained diabase were more developed.In addition,thin and fine-grained diabase strongly modified by tectonic activity can also form high-quality reservoirs,That is,the improvement of fractures on the reservoir even compensates for the adverse impact of lithology.