Reservoir Characteristics And Formation Mechanisms Of The Sinian Dengying Formation In Southern Sichuan To Northern Guizhou,China

The Southern Sichuan-Northern Guizhou area is a high potential area for oil and gas exploration in the Sichuan Basin,and the Sinian marine carbonate rocks have abundant hydrocarbon resources.Although many scholars have made remarkable research achievements from Dengying Formation in this region,the relative lack of outcrop and drilling data,complex diagenesis and heterogeneity restrict the research of the formation mechanism of high-quality reservoirs in the Dengying Formation.An integrated study based on petrography,facies,petrophysical property,cathodoluminescence,stable isotope,elements and X-ray diffraction analyses from outcrop and core samples was performed to provide insight into the characteristics,diagenetic evolution,formation mechanism and controlling factors of the Sinain Dengying dolomite reservoirs.（1）The Dengying Formation is dominated by a restricted platform environment in the study area.The lithologies of the Dengying Formation mainly include microbial dolomite,granular dolomite and crystalline dolomite,etc.Some of these dolomites contain a small amount of mudstone,gypsum salt rock,siliceous rock and limestone.Five sub-facies and fifteen microfacies have been recognized in the Dengying dolomites based on a detailed analysis of the core and outcrop observation,sedimentary facies,structure and structural characteristics:microbial mound,granular beach,platform,lagoon and depression,respectively.Among them,the Deng-1 and Deng-3 members are mainly composed of platform,lagoon and depression.The Deng-2 and Deng-4 members are characterized by wide development of microbial mounds and granular shoal in the inner platform,and these types of sub-facies are conducive to the development of Dengying Formation reservoirs in southern Sichuan and Northern Guizhou.Therefore,we have established the depositional models of Deng-2 and Deng-4 members,respectively.（2）Based on the outcrop and core observation,and combining analytical data of carbon and oxygen isotopes,strontium isotopes,major and trace elements,cathodoluminescence and order degree from the fabric-preserving dolomite samples,we unravel the genetic mechanism of the Dengying dolomites.The microbial dolomite samples have the highestδ¹³C values,slightly elevated ⁸⁷Sr/⁸⁶Sr values,the lowest Fe and Mn content and relatively low order degree,indicating that these dolomite samples are the product of microbial dolomitization during the syngenetic diagenetic stage.The micritic dolomite samples have relatively lowδ¹³C values,the highestδ¹³O values,slightly depleted ⁸⁷Sr/⁸⁶Sr values,the highest Na,K,Sr contents and the lowest order degree,indicating that it is the product of evaporation pump in a concentrated brine environment with high salinity.The granular dolomite samples have the lowestδ¹³C,δ¹³O and ⁸⁷Sr/⁸⁶Sr values,the lowest Na,K,Sr contents,and the highest Mn content and order degree,reflecting the genetic characteristics of reflux dolomitization during the penecontemporaneous diagenetic stage.（3）Four main lithologic types were identified in the dolomite reservoirs of the Dengying Formation,and include dolarenites,thrombolytic dolomites,stromatolite dolomites and powder to medium crystalline dolomites.The reservoirs spaces are composed of various types of secondary pores and can be mainly divided into five types intragranular solution pores（mold pores）,intergranular solution pores,intergranular（solution）pores,karst fractures and cavities,and dissolution fractures.The average matrix porosity and permeability of the reservoirs are 2.42%and0.42×10^-3μm²,and shows typical features such as low porosity and permeability.The Dengying Formation in the study areas formed a complex pore system in a long-term diagenetic alteration process since deposition,which caused intensification of the heterogeneity of reservoir and change the formation and distribution of these reservoirs.（4）Based on the above comprehensive analysis of stratigraphy,sedimentology,dolomite genesis,reservoir characteristics and diagenetic evolution,and combined with the analysis data of carbon,oxygen and strontium isotopes,trace elements and cathodoluminescence,we considered that the Dengying Formation has been transformed by four stages of dissolution:meteoric water leaching,supergene karstification,organic acid dissolution and hydrothermal alteration.The meteoric water leaching is generally associated with the high-frequency sea-level fluctuation during the penecontemporaneous stage.This type of dissolution has typically characterized by fabric-selective dissolution,in which its intensity is weak and the range of action is restricted.The supergene karstification mainly occurred in the surface or near-surface diagenetic environment as a result of long-term tectonic uplift,which is characterized by the formation of a large scale of non-fabric selective karst fractures and caves system.Meanwhile,many fillings and dissolution relicts also exist in these karst fractures and caves,such as dolomite,quartz,bitumen,karst breccia and vadose silt.The organic acid dissolution is related to the thermal evolution of organic matter.It usually occured in a relatively enclosed medium-deep buried diagenetic environment,and mainly formed intergranular solution pores and intergranular solution pores with non-fabric selective dissolution characteristics,and residual asphaltene is common around these pores.In addition,the Dengying Formation is also affected by hydrothermal activities caused by taphrogenesis during the medium-deep burial diagenetic stage.In the process of hydrothermal alteration,the formation of dissolution pores is also accompanied by the development of hot cataclasm breccia,banded structure and hydrothermal minerals（especially saddle dolomite cement）.The various types of secondary pores formed by the four stages of dissolution composed the main reservoir space in Dengying Formation.（5）Development and distribution of high-quality reservoirs of the Dengying Formation in the Southern Sichuan-Northern Guizhou region were essentially controlled by sedimentary facies,diagenesis and tectonism.Sedimentary facies were closely related to the development of high-quality reservoirs,and belong to dissolution-alteration type reservoirs controlled by sedimentary facies.The granular shoal and microbial mound have higher porosity and permeability relative to platform flats and other subfacies,which is the material basis for reservoir development in Dengying Formation.The reservoir quality is greatly modified during the diagenetic evolution.The matrix porosity of the Dengying Formation is greatly hindered by compaction,multi-stage cemented filling and silicification,resulting in the significant decrease of the physical properties of the reservoir.In contrast,dolomitization,recrystallization and multi-stage dissolution promoted the development and preservation of high-quality reservoirs,among which the multi-stage dissolution is paramount important.The superposition of supergene karstification and meteoric water leaching can significantly increase the total porosity of the reservoirs.During the medium-deep burial diagenetic stage,the combination of organic acid dissolution and recrystallization effectively adjust and redistribute the existing pore structures,which improve the physical properties of reservoirs.In the process of hydrothermal alteration,while carbonate components were dissolved by hydrothermal fluids,some existing dissolution pores were also blocked by hydrothermal minerals,resulting in the changes in the development and distribution of high-quality reservoirs in the Dengying Formation.Most of the structural fractures formed by tectonism have been transformed by dissolution or blocked by multi-stage authigenic minerals.At present,the effective structural fractures are mainly formed in the tectonic uplift movement during the Himalayan period,which is of positive significance to enhance the permeability of the reservoir.