Regional Retention Condition And Its Assessment Of The Longmaxi Formation Shale Gas At The Southeast Margin Of Sichuan Basin

The Lower Silurian Longmaxi Fm.organic-rich shale(LSOS)developed at the southeast margin of the Sichuan Basin(SMSB)is currently the only set of marine shale in which commercial breakthroughs have been achieved in China.Previous studies have demonstrated that the LSOS in this area was formed under various paleogeographic settings,which led to the significant differences among the material basis of the shales,such as the mineral composition and source rock properties.Moreover,influenced by the multi-stage and long-term tectonic movements,the LSOS was deeply buried during the early phase of its evolution,resulting in its relatively high thermal maturity.The LSOS then experienced intensive tectonic compression and massive uplifting and erosion during the late phase of its evolution.The self-sealing of the LSOS was compromised due to the well-developed deformations and rock failures of shale.The pre-reserved shale gas dissipated through all kinds of pathways,including the faults,fractures,and the denuded zone on the ground surface.The actual exploration practice has proved that the retention condition of the shale gas is the most crucial factor which controls the commercial values of the LSOS at the SMSB.However,most of the existing studies on the retention condition of the LSOS shale gas have not got to the nature of the shale gas retention,i.e.,the joint research between the internal properties of the shale and the variations of its external setting,as well as the static elements and dynamic processes of the shale gas retention.Meanwhile,the scales of these studies were usually limited,and they were not quantitative and predictive enough either.Given this,the LSOS developed at the SMSB was chosen to be the study's objective in this dissertation.The two aspects of the shale gas retention,i.e.,the internal properties of the shale and the variations of its external setting,were focused.The regional retention conditions of the LSOS shale gas were studied and evaluated from several aspects,which were the material basis,burial evolution,characteristics of the deformation and rock failure,lateral shale gas conduction,and dissipation of the LSOS,etc.Several major insights can be gained:(1)Based on the measured data on wells and outcrop sites,the spatial distribution,depositional facies,and material basis of the LSOS were studied based on sedimentology,petromineralogy,and petroleum geology theories.The study results suggested that the LSOS at the SMSB was mainly developed within the 1^st sublayer of the lowest member of the Longmaxi Fm.The depositional facies of most LSOS were deep marine shelf facies,while the minor part of the LSOS developed in the areas near the paleo uplifts were shallow marine shelf facies.The LSOS is the typical argilliferous siliceous shale,which is characterized by a total content between 55%and 65%of the felsic minerals,a total content between 25%and 30%of the clay minerals and a total content between 10%and 15%of carbonate minerals.The total thickness of the LSOS is between 5 meters and 40 meters,of which the majority is over 20 meters.The total carbon content of the LSOS is between 1.0%and 4.0%,of which the majority is over 2.0%.The depositional facies of the LSOS control its mineral composition,total thickness,and total carbon content,and its source rock property and engineering performance are excellent in general.(2)Based on the measured data on wells,regional geological maps,and the low-temperature chronology and geothermal study results extracted from the published documentations,the 1D basin modeling technique was utilized to study the current burial status and burial evolution of the LSOS at the SMSB.The results showed that the current status of the LSOS varied from being buried,exposed on the ground surfaces to being eroded at different tectonic locations,in which the maximum buried depth could reach about 6 000 meters.The(equivalent)vitrinite reflectance of the LSOS is between 1.5%and 3.9%,of which the majority is over 2.0%,indicating a high-over maturation.Affected by the Yanshanian and Himalaya Movement,the LSOS experienced significant uplifting and erosion during the Mesozoic and Cenozoic Eras.The onset time of the uplifting spanned from ca.180 Ma at the southeastern orogenic belt to ca.70 Ma at the northwest stabilized tectonic belt inside the Sichuan Basin.The amplitudes of the uplifting and erosion also showed a decreasing tendency along the SE-NW direction.The LSOS entered the hydrocarbon generation threshold(Ro=0.5%)at the end of the Early Paleozoic Era and gradually stepped into the main zone generating wet gas(Ro=1.3%).The main zone generating oil-cracking gas(Ro=2.0%)was passed at the Early Mesozoic Era.The initiation and ceasing of the thermal maturation and hydrocarbon generation were much earlier in the LSOS outside the basin than inside the basin.At the same time,the LSOS outside the basin was also uplifted and eroded much earlier,which led to significant disadvantages on both hydrocarbon generation and retention.(3)The shale core specimens with various mineral compositions and the intact brittle rock specimens were recovered from wells and the outcrop sites,respectively.Triaxial compression rock mechanical tests and the acoustic emission tests based on the Kaiser effect were performed using these rock specimens,respectively.The internal rock mechanical properties of the LSOS and the external stress settings experienced by the LSOS during the Mesozoic orogeny and Cenozoic uplifting were studied according to the test results integrated with the classic elastic mechanical theories.The results suggested an obvious negative linear correlation between the triaxial mechanical strengths of the shales and their total clay mineral contents.At the same time,there was only a weak positive correlation between the strengths of shales and their total carbonate mineral contents.No apparent correlation between the strengths of shales with their total contents of felsic minerals was observed.Furthermore,the total clay mineral content of shale could be utilized to represent the correlation between the in-situ mechanical strength of shale and the confining pressure taken by the rock frame of shale.The SMSB experienced intensive horizontal tectonic compression during the Mesozoic orogeny.The magnitudes of the paleo-effective maximum horizontal stress ranged from 40 MPa to 180MPa,which were decreasing along the SE-NW direction and positively correlated with the dip angles of the strata.The ratios between the horizontal stress magnitudes taken by the shale and its in-situ triaxial mechanical strength could be employed as the indicators of the possibility of rock failure during the tectonic compression and uplifting.These indicators suggested that the LSOS near the southeastern orogenic belt outside the basin,as well as the LSOS,developed within cores of the chevron anticlines inside the basin,have higher possibilities of rock failure.(4)A 2D petroleum geological model of the Sangzheping Syncline was built based on the elaborate reconstruction of its tectonic evolution using the data from wells,3D seismic,and regional geology maps.The burial,thermal-maturation,hydrocarbon generation,and conduction histories within the LSOS system were rebuilt using the 2D basin modeling technique.The factors influencing the lateral conduction and dissipation process of shale gas were discussed according to the comparison of the simulation results using the 2D models in which various dip angles of strata and total durations of the uplifting were input.The modeling results indicated that the shale gas in the LSOS system was primary originated from the secondary cracking of the generated oil.During the whole process of deformation-uplifting-erosion from the maximum burial of the LSOS to present,the shale gas gradually conducted laterally from the core of the syncline to its wings,driven by the potential differences of elevation.After the LSOS developed within the wings was exposed on the ground surface,the shale gas thereupon dissipated,resulting in a significantly higher gas content at the core than the wings.The longer the total duration of uplifting and the formation dip angles are,the stronger the lateral dissipation will be,and the less shale gas could be retained within the syncline core.(5)Based on the above understandings and results of the material basis,burial evolution,regional deformation,and rock failure characteristics of the LSOS as well as the lateral dissipation process of the shale gas therein,a total of nine indicators in three categories,including the total thickness,total organic carbon content,and vitrinite reflectance of the LSOS,the rock failure factors of tectonic compression and uplifting,total duration of uplifting,formation dip angles,distance from the exposed zones of the LSOS and exposed faults,were selected.A corresponding criterion was also established to facilitate the quantitative evaluation of the regional retention condition of the LSOS.The selected areas which possessed the excellent,acceptable,risky,and unacceptable retention condition of shale gas accounted for 8.91%,26.15%,13.68%,and 51.26% of the total area,respectively.