Development Mechanism Of Ordovician Carbonate Paleokarst In The Halahatang Area Of The Tarim Basin

Global research into marine carbonate paleokarst reservoirs has been picking up pace.Because of the highly heterogeneous structure of carbonate karst formations,their exploration and development as oil and gas reservoirs proves to be extremely difficult.At the time of writing,279 carbonate reservoirs distributed in 39 countries and regions around the world have been discovered.The reserves amount to about 50%of the world's total oil and gas resources,and the output accounts for more than 60%of global production.With the recent rapid development of the petroleum industry in China,the exploration and development of marine carbonate oil and gas fields are at a critical stage.Moreover,the focus of exploration and development of carbonate oil and gas fields in China has transformed from the identification of strategic petroleum reserves to reserve replacement.Carbonate paleokarst and oil and natural gas resources are closely related,since dissolution pores,holes,and cracks,which are typical of karst regions,are often important features in oil and gas migration and accumulation.Based on statistical data,carbonate rocks,which account for 20%of the total amount of sedimentary rocks,contain over 50%of the world's proven oil and gas reserves.As a result,multidisciplinary study of carbonate paleokarst reservoirs has been gaining increasing attention from researchers in the fields of karst hydrogeology and petroleum geology.Experts and scholars have conducted a significant amount of research on oil and gas reserves in paleokarst reservoirs,focusing primarily on the conditions required for oil and gas accumulation,migration,storage,and charging.Moreover,a significant number of studies have also been conducted on paleokarst reservoirs,focusing mainly on fracture and dissolution pore evaluation,surface and underground karst development modeling,seismic reflection and image logging,paleokarst identification by borehole drilling and electrical logging,determination of the geochemical characteristics of paleokarst fracture-fill material,and similar methods that can reveal paleokarst characteristics and paleoenvironmental conditions.However,relatively few studies have focusedonkarstpaleogeomorphologicalrestorationmethods,karst paleogeomorphology or drainage system visualization,karst underground river identification or hydrodynamic analysis of various paleolandforms,or geological modeling of regional karst evolution based on paleokarstification mechanisms,and the geochemical properties of karst-filling material.The carbonate reservoirs of the Tarim Basin oil and gas fields have undergone a lengthy karstification process.The long geological history of karstification in surface and near-surface environments,as well as the superposition and transformation during the burial karstification process,resulted in the development of very complex paleokarst fracture-cavity systems,which are highly heterogeneous.In addition,the Ordovician carbonate paleokarstic fracture-cavity reservoirs are buried very deeply;for example,the Lunan buried hill is buried to a depth of about 4500-5500 m;the main area of the Tahe Oilfields,to about 5000-6000 m;and the Halahatang Oilfields,to about5000-7000 m.At present,the maximum measured?not vertical?drilling depth in Ordovician carbonates in the Shunbei area of the Tahe Oilfield is 8250 m.Hence,the exploration,development,and prediction of paleokarstic reservoirs are extremely difficult.The variations in paleokarstic geomorphology and hydrodynamic conditions resulted in changes in the paleokarst and karst-fill characteristics,and also restricted the distribution and types of karst reservoirs.This paper classifies the Ordovician carbonate karst formations and identifies paleokarstification during the Ordovician depositional hiatus,based on geochemistry and geological background analyses.Using various paleogeomorphological restoration methods,we carried out restoration of three-phase depositional-hiatus paleokarst surfaces,and identified second-and third-order,and a number of micro geomorphological units.Next,we carried out paleodrainage system visualization,including surface water and subsurface river systems in the interbed karst-bedding reform zone of the Lianglitage Formation.The paleogeomorphology and paleo-water system were verified based on combined seismic attributes.After conducting an in-depth study of the distribution patterns of the paleokarst reservoir space,and of the characteristics of stable carbon and oxygen isotopes and calcite inclusions in the paleocarbonate fracture-cavities,we proposed various geomorpholigical and paleo-hydro-dynamic characteristics and environmental conditions,and established karst reservoir models for different zones and a regional karst geological evolution model.We also discovered the development mechanism of the Ordovician carbonate paleokarst in the Halahatang area of the Tarim Basin.We obtained the following results,which led us to the below-mentioned conclusions.This study first focused on the geological background of karstification in the Halahatang area.We found that the Ordovician carbonate rocks exhibited three phases of sedimentary hiatus paleokarstification,including not only the buried-hill?weathering crust?karstification,but also the interlayer karstification superposition and transformation.The Ordovician carbonate strata in the Halahatang area pinch out successively northward.The geological structure of the study area is complex,and its structural fractures are mainly NE-SW and NNW-SSE strike-slip faults.Based on the results of geochemical analysis,the Ordovician carbonate karst formations in the Halahatang area were divided into three classes,four types?relating to karstification period and environment?,and six subtypes.On this basis,in connection with the complex geological formations of the three karstification phases of the Ordovician,we proposed the"relative residual thickness and paleo-stratigraphic trend-surface"combination method and the"residual and residual trend-surface"method,and used them to respectively restore the paleokarst geomorphology of the Lianglitage and the Yijianfang formations.We used the conventional"impression"method to restore the paleokarst geomorphology of the Pre-Serulian.Based on modern karst theory,we identified second-and third-order geomorphological types.Hence,we identified paleokarst water systems of different periods,including surface water and underground river systems,and verified the results by comparing them with the seismic reflections of the geological formations.Moreover,we carried out an in-depth analysis of the relationship between hydrodynamic conditions and karstification processes during various paleokarst periods.Because different paleokarst geomorphologies have varied hydrodynamic properties,we determined differences in the development patterns of the karst fracture-cavity systems and the fill-material characteristics.Based on the Lianglitage paleokarst geomorphology and hydrodynamic field characteristics,the karst formation characteristics,the paleokarst fracture-cavity system development mechanisms,and the combined characteristics of the surface river cutting depth and underground river development,we divided the Halahatang area into four karst zones.We carried out an in-depth study of the development pattern of the paleokarst fracture-cavity system of the various karst zones and of the paleokarst development mechanisms.Based on the measurement and analysis of stable carbon and oxygen isotopes of calcite-filled inclusions of paleokarst fracture-cavities,we studied the hierarchical significance of the karstification periods and the paleoenvironments of the study area.The carbon and oxygen isotope analysis of calcite showed that the range of?¹³C values?PDB?was relatively large,namely,from 2.12‰to-4.09‰,with a mean value of-0.65‰,while the?¹⁸O values had a negative range,namely,from-4.14‰to-15.71‰,with a mean value of-9.95‰.The results reflect the four karstification periods and paleoenvironmental characteristics.The analysis of the paleokarst fracture-cavity filling demonstrated that calcite inclusions can be divided into three types,based on their salinity levels:low-,medium-,and high-salinity inclusions.Based on the homogenization temperature,the main formation stages of inclusions can be divided into four temperature ranges.These results indicate that a total of four large-scale paleokarstification and filling periods existed:in addition to the Caledonian period,which was characterized by the weathering-crust meteoric-water exposed karst,we also observed the Late Hercynian buried-karst period,the Indo-Yanshan buried-karst period,and the Himalayan buried-karst period.These findings are consistent with the results of the carbon and oxygen isotope analyses.We established four geological models for karst reservoir formation?the buried-hill karst zone,the interbed karst-bedding reform zone,the interbed karst-platform margin overlap zone,and the interbed karst fault-controlled reservoir zone?through in-depth study of the paleogeomorphology and paleotopography of the reservoir and correlative units,and analyses of the geological background of karstification,periods of karstification,paleokarst geomorphology,and paleohydrodynamic conditions.Moreover,the regional geological evolution models for various paleokarstification periods revealed the development mechanism of the Ordovician carbonate paleokarst in the Halahatang region.The results of this study provide a new direction for research into carbonate paleokarst-type oil and gas reservoirs.