Study On Characteristics And Distribution Of Ordovician Fault-controlled Reservoirs In Tahe Oilfield

Carbonate reservoirs plays an important role in petroleum exploration and development.Proven reserves of carbonate reservoir account for 60%of the world's total proven reserves.The distribution of reservoirs is the most important part of carbonate reservoir research.At present,the carbonate reservoirs formed by sedimentary are highly studied.For the deep karst cave carbonate reservoirs,due to deep burial,diversity of reservoir types,complex origin and strong heterogeneity,it is difficult to predict this kind of reservoirs,and lack study of the formation mechanism and distribution of karst.Tahe Oilfield,as a typical deep buried hill karst fractured-cave carbonate oilfield in China,focuses more study on weathered crust fractured-cave body controlled by the plane of unconformity and groundwater table.Meanwhile less study on karst reservoirs in buried hill slope area and deep inner area.The formation and distribution of these reservoirs are not only related to the weathering crust karstification,but also controlled by the fracture effect and its difference.Thus,this paper takes the Ordovician carbonate reservoir in Tahe Oilfield of buried hill slope area as the research object,under guidance of theories and methods of carbonate rock petrology and sedimentology,karst geology and geomorphology,tectonic geology,geochemistry,geophysics,hydrogeology etc.,comprehensive utilization of seismic,logging,core,analysis of test,production performance and similar outcrops data,this paper focus on the differences of fault system and the internal structure of the fault zone on the control of karst reservoirs to discuss the distribution law and development model of fault controlled karst reservoirs.Main research results as following:?1?Starting from the fracture factor of controlling karst reservoirs,the differences of the Ordovician fault system in the Tahe Oilfield are analyzed in the form,period,grade and segmentation of the fault.Based on the internal structure of similar outcrop fault zone,four internal structures of fracture zone are proposed.The identification of fault zone is realized by using logging method and seismic technique.There are great differences in structure and physical properties of fault zone at different periods of fracture evolution and different location in the same fault,and the evolution model of fault zone is established.?2?For the problem of complex fluid sources for deep karst interactions,The study of regional tectonic evolution history,mineral combination analysis by thin section,carbon,oxygen and strontium isotope analysis,fluid inclusions homogenization temperatures analysis as the basis to clear that there are three kinds of fluid types such as atmospheric fresh water,hydrothermal fluid and formation water in the Ordovician carbonate karst in Tahe Oilfield.And from the discussion of dissolution mechanism of different fluid types,it shows that atmospheric freshwater dissolution is the most important karst effect,which is mainly controlled by the condition of freshwater recharge with CO₂,and the volcanic hydrothermal fluid activity releases a lot of acid gas and thermal energy with certain dissolution function such as CO₂,H₂S,SO₂ to promote the thermochemical reduction of sulfate.The model of atmospheric freshwater karst and the model of hydrothermal fluid karst were established.?3?It is difficult to identify and predict fracture-cave reservoirs from seismic cube,especially the isolated karst reservoirs cause the complex seismic reflection field.Under the guidance of geological factors of karst cave reservoirs,based on palaeo-geomorphology restoration and forward modeling.A method based on seismic waveform inversion is proposed to quantitatively identify the spatial distribution and lateral distribution of different types of reservoirs.At the same time,a carving method of fracture-cave body based on tectonic attribute seismic data body is formed,which realizes the fine characterization of the karst reservoir in coverage area.?4?This paper discussed the reservoir forming and controlling mechanism of fault controlled karst reservoirs and its evolutionary models.Grade?fault and its associated grade?-1 fault controls the favorable location of karst reservoir development in fault crossing.The difference of internal structure and tectonic pattern of fault zone controls the development scale of karst reservoir.Considering the tectonic evolution,the difference of fracture system,the type of karst fluid and the coupling relationship between fracture and fluid of Tahe Oilfield to establish the fault controlled karst evolution model in three important periods in the middle of Caledonian,early Hercynian and late Hercynian.In the early Hercynian,the surface freshwater flows downwards along the fault zone,which is the most important period of the formation of the controlled karst reservoir.The deep hydrothermal fluid flows in the fault zone under the"seismic pump"in the late Hercynian,which is the main period of hydrothermal karst development.?5?Based on the influence of fracture and other factors of the karst reservoir development to establish the fault controlled karst reservoir development model of fractured-dark river type,fault-controlled isolated cave type,fault-reef beach facies coupling type,fault-palaeogeomorphology-paleo-surface water system coupling type etc.and to study the distribution of different types of fault controlled karst reservoirs.The research results enrich and deepen the research on the genesis of carbonate reservoirs,which have important theoretical value and practical significance for deep exploration and development of carbonate reservoirs in Tarim Basin.