Microscopic Characteristics And Diagenetic Mechanisms Of Deep-burial Carbonates

Deep-burial carbonates are considered a reliable succession for oil and gas production.However,the long-term complex geological processes have left the reservoir development characteristics and the diagenesis mechanisms to be further refined.This thesis takes the Middle-Lower Ordovician carbonates of the northern slope of the Tazhong Uplift in the Tarim Basin as the object of study.Combined scanning electron microscopy(SEM)and nuclear magnetic resonance(NMR)tests are used to characterise the reservoir space of widely distributed microporous carbonates.Integrated petrological and multiple types of geochemical evidence,including in situ rare earth elements(REE+Y),are considered for the genesis mechanism of locally-developed siliciclastic and dolomitic rocks.The reservoir development patterns of deep-burial microporous carbonates and hydrothermally modified carbonates have been established respectively through the analysis of roles of diagenesis in controlling reservoir development.The carbonates in the study area are buried at a depth of more than 6,000 metres,with porosity generally not exceeding 5%,permeability generally less than 10 m D.Over 95%of the reservoir space consists of micropores existing between calcite or dolomite microcrystals with a radius of no more than 10?m,typical of deep-burial microporous carbonates.The micropores can be divided into three categories based on crystal morphology and intercontact relationships.The first type of micropore is hosted between subhedral to anhedral microcrystals with distinct boundaries and is the predominant reservoir space for the microbial lime-/dolostone,generally corresponding to relatively high porosity(averaging 3.41%).The second type of micropore is found between microcrystals with coalescent polyhedral to anhedral structures that are blurred at the boundaries between them,and is the dominant reservoir space for abiotic limestone.The third type of micropore is present between the rhombic dolomite crystals.The influence of different diagenetic processes on the development of deep-burial carbonate reservoirs is summarised and the origin of hydrothermal fluids is clarified.Long-term strong cementation and compaction have resulted in a convergence of porosity and permeability values in grainstones,packstones,wackestones and mudstones.The‘Ostwald ripening'effect increases pore space whereas the limited cementation maintains pore space.Magma-sourced hydrothermal fluids,rather than basinal brines heated by magmatism,are responsible for precipitation of cherts.Deep-seated,ultramafic-derived hydrothermal fluids and/or magmatic fluids associated with Permian volcanism may have been involved in the formation of hydrothermal dolomites in the study area.Accompanying formation and reactivation of the strike-slip faults on the northern slope of the Tazhong Uplift,small volumes of hydrothermal fluids might have been triggered by seismic pumping.The early formed strike-slip fault systems may also have served as conduits for the subsequent large-scale transport of deep-seated,ultramafic-derived thermal fluids and/or Permian magmatic fluids,along which fluids were transported upwards at high flow rates.Carbonation of ultramafic rocks through interaction with deep-seated hydrothermal fluids rich in CO₂ may have served as a magnesium and silica source for dolomitization and silicification.Two types of deep-burial carbonate reservoir development patterns were established separately.Based on the bimodal distribution in grain size of the larger authomorphic calcite crystals and the smaller spherical microcrystals encapsulated within them,it is hypothesized that the dissolution-precipitation effect caused by‘Ostwald ripening'is responsible for the longitudinal cyclonic variation in porosity in the Yijianfang Formation.Hydrothermal fluid controls on deep carbonate reservoirs are spatially and temporally heterogeneous.Hydrothermal fluids are dominated by dissolution in the early stages and precipitation in the later stages.Hydrothermally modified carbonate reservoirs mainly distributed along the fracture zones.