Evolution Of Porosity And Permeability Of Deep Carbonate Reservoirs In Superimposed Basins And The Lower Limit Of Oil Accumulation

China's shortage of oil and gas drives the continuous exploration of deep formations.More than 90%of the new reserves in Tarim come from deep formations(more than 6,000 m),with average depth of discovered oil and gas reservoirs exceeding 6,043 m.The deepest exploratory well and oil reservoir in China reache carbonate formations at 8,882 m and 8,408m,respectively.Though previous studies thoroughly investigated the lowest limit of hydrocarbon in clastic rock reservoirs,a gap of knowledge remains on carbonate reservoirs.The exploration of deep carbonate rocks in the Tarim Basin shows that even though good hydrocarbon reservoirs still exist below 8,000 m in some wells,it turns to be dry reservoirs when the porosity of the reservoirs decrease to a small value.Is there also a lower limit of hydrocarbon occurrence in carbonate reservoirs?If so,what is the critical condition?How to characterize it?What is the generating mechanism?These questions remain to be puzzles for petroleum geologists.China's growing focus on deep formation oil and gas exploration adds great significance to the answer of these questions,as it benefits in predicting the favorable exploration fields of deep carbonate oil and guiding the deep oil drilling.The Tarim Basin is a typical superimposed basin.Deep oil exploration in the Tarim Basin is at the forefront of the world,and the oil exploration is mainly carbonate reservoirs.Therefore,the oil accumulation in carbonate reservoirs of Tarim Basin is used as an example to study.This research is based on the IHS(IHS Markit)database of logging data from 6,373 carbonate reservoirs in the world,2,778 carbonate reservoirs of Volga-Urals Basin in the Russian,and more than 5,708 deep carbonate reservoirs of superimposed basins(Sichuan Basin,Tarim Basin)in western China.355 carbonate rock cores were taken for mercury intrusion experiments,and 500 source rock samples were used for rock pyrolysis experiments.The evolution of porosity and permeability of deep carbonate reservoirs and the lower limit of oil accumulation in the Tazhong area of Tarim Basin were studied using geological analysis,statistical analysis,experiments,and numerical simulation.Followings are three main achievements:First,the porosity and permeability characteristics of carbonate reservoirs at China and abroad are compared and analyzed according to IHS(IHS Markit,2020)data.The analysis of global carbonate reservoirs shows a completed but generally negative relationship between the porosity and the permeability of carbonate reservoirs and the burial depth.All reservoir porosity and permeability parameters(P90,P50,P10 and Maximum)present similar decreasing trends with increasing reservoir burial depth,indicating that a lower limit of hydrocarbon accumulation also exist in carbonate reservoirs.The comparison of domestic and foreign carbonate reservoirs shows that foreign reservoirs have higher porosity and permeability and better correlations to burial depth,while in domestic carbonate reservoirs,the porosity and permeability are generally low and correlate poorly with burial depths.Second,a numerical characterization method for the particularity of carbonate reservoirs in China's superimposed basins has been established and the lower limit for oil accumulation of Lower Ordovician carbonate reservoirs in the Tazhong area of the Tarim Basin have been proposed.This research established a mathematical model to simulate the change of physical properties with the buried depth for carbonate reservoirs,and the change of porosity and density of the Lower Ordovician carbonate reservoirs with the buried depth in Tazhong area is simulated with Matlab.The simulation shows that the porosity decreases and the density increases with the buried depth,which well alines with site measurements.The physical properties of carbonate reservoir are mainly controlled by five dominant geological factors,including geological age,dynamic deformations,depth of burial,formation temperature and homogeneity.The porosity of the carbonate reservoir is relatively small in geological conditions with higher depth,more dynamic deformations,higher geological age,higher temperature gradient and higher homogeneity.Oil-bearing layer ratio method,minimum flow pore-throat radius method,and dry layer ratio methods were combined to determine the critical condition for the lower limit of oil accumulation of Lower Ordovician in Tazhong area,Tarim Basin.The critical condition is 1.8%porosity,0.07 mD permeabilit,and 0.01 ?m pore throat radius.Third,the formation mechanism of the lower limit of oil accumulation is discussed.This research revealed two dominant mechanisms of the lower limit of oil accumulation in carbonate reservoirs.1)The decrease of capillary pressure difference with burial depth leads to the end of hydrocarbon accumulation;2)the depletion of hydrocarbon generation potential in source rocks with buried depth leads to the end of hydrocarbon accumulation,which represents the occurrence of the lower limit of oil accumulation.Based on those mechanisms,the oil accumulation depth limit of the lower Ordovician deep carbonate reservoirs in the Tazhong area was determined to be 9,000 m?9,200 m.The critical porosity of the lowest limit of oil accumulation correlates with the depth limit,corresponding to the 9,000 m depth.Four geological factors-geological age,dynamic deformations,depth of burial,formation temperature and homogeneity,alter the oil accumulation depth limit.The oil accumulation depth limit is shallower in geological condition with more dynamic deformations,old geological age,higher temperature gradient and better homogeneity.To our present knowledge,the maximum burial depth of the Lower Ordovician carbonate reservoirs does not exceed 8,000 m in the Tazhong area,making it feasible and promising to conduct deep carbonate oil exploration.