Factors controlling the formation and evolution of source rocks in the Shahezi Formation,Xujiaweizi fault depression,Songliao Basin

The types and quality of source rocks in the Shahezi Formation are the key factors affecting the distributions of various deep gas reservoirs in the Xujiaweizi fault depression in Songliao Basin.To clarify the quality differences and origins of different types of source rocks in the Shahezi Formation,this study reconstructed the sedimentary and water environment,determined the controlling effects of fault activity,sedimentary facies,and paleo-sedimentary environment on the quality of various source rocks,by making full use of seismic,logging,core,organic geochemical and element geochemical analysis.The results show that two types of source rocks developed in the Shahezi Formation,namely,mudstones and coals.The mudstones have a relatively high abundance of organic matter,which consists of type-Ⅱ kerogen and partial type-Ⅲ kerogen,and are concentrated in Sha-I Member.The coals have a high abundance of organic matter,which consist of type-Ⅲ kerogen,and are mainly distributed in Sha-Ⅱ Member.During the deposition of Sha-I Member,intense fault activity formed arrow,deep-water lacustrine basins with high salinity and strong reducibility on the downthrow sides of faults.During the deposition of Sha-II Member,fault activity progressively weakened,and the areas of lacustrine basins enlarged to their maximum values and became wide,shallow-water basins with low salinity and low reducibility.The development of source rocks was controlled by fault activity,sedimentary facies,and paleo-sedimentary environment.Fault activity formed accommodation space on the downthrown sides of faults for mudstone accumulation,thus determining mudstone thickness.The sedimentary environment controlled the organic matter input and determined the distribution of mudstones and coals.The paleo-sedimentary environment,which consisted of paleo-salinity,as well as paleo-water depth and redox conditions,affected the accumulation and preservation of organic matter and is the main controlling factor for the quality difference of various source rocks in the Shahezi Formation.

Accumulation and exploration of continental shale gas resources of Cretaceous Shahezi Formation in Lishu fault depression,Songliao Basin,NE China

Distribution characteristics,organic matter development characteristics,gas-bearing characteristics,reservoir characteristics,and preservation conditions of the Shahezi Formation shale of Lower Cretaceous in the Lishu fault depression,Songliao Basin,NE China,are analyzed using organic geochemical,whole rock,and SEM analysis data,and CO_(2)and N_(2) adsorption and high-pressure mercury injection experiment data in combination with the tectonic and sedimentation evolution of the region to reveal the geological conditions for enrichment and resource potential of continental shale gas.The organic-rich shale in the Lower Cretaceous of the Lishu fault depression is mainly developed in the lower submember of the second member of the Shahezi Formation(K_(1)sh_(2)^(1) Fm.)and is thick and stable in distribution.The shale has high TOC,mainly types II_(1) and II_(2) organic matter,and is in the mature to the over-mature stage.The volcanic activity,salinization,and reduction of the water environment are conducive to the formation of the organic-rich shale.The shale reservoirs have mainly clay mineral intergranular pores,organic matter pores,carbonate mineral dissolution pores,and foliated microfractures as storage space.The pores are in the mesopore range of 10–50 nm,and the microfractures are mostly 5–10μm wide.Massive argillaceous rocks of lowland and highstand domains are deposited above and below the gas-bearing shale separately in the lower submember of the K_(1)sh_(2)^(1) Fm.,act as the natural roof and floor in the process of shale gas accumulation and preservation,and control the shale gas enrichment.Based on the above understandings,the first shale gas exploration well in Shahezi Formation was drilled in the Lishu fault depression of Songliao Basin.After fracturing,the well tested a daily gas production of 7.6×10^(4) m^(3),marking a breakthrough in continental shale gas exploration in the Shahezi Formation(K1 sh Fm.)of the Lishu fault depression in Songliao Basin.The exploration practice has reference significance for the exploration of continental shale gas in the Lower Cretaceous of Songliao Basin and its periphery.

Methane adsorption effected by pore structure of overmature continental shale:Lower Cretaceous Shahezi Formation,Xujiaweizi Fault,Songliao Basin

Overmature continental shale is commonly developed,but few studies have given insight into its pore structure and sorption capacity.Various techniques,including SEM,helium porosity and permeability,N_(2)/CO_(2)adsorption,MICP,and NMR,were used to detect the pore structure of shale from the Shahezi Formation,Xujiaweizi Fault,Songliao Basin.The excess methane adsorption volumes were measured by the volumetric method and modeled by the Langmuir model.Based on the findings,the most developed pores are intraparticle pores in clay minerals,followed by the dissolution pores in feldspar,but organic pores are uncommon.The selected shales have low helium porosity(mean 1.66%)and ultralow permeability(mean 0.0498×10^(−3)μm^(2)).The pore throats are at the nanoscale,and the pore-throat size distributions are unimodal,with most less than 50 nm.The studied shales are characterized by the lower specific surface area(SSA)and pore volume(PV)but the larger average pore diameter.The total SSA is contributed by the micro-and mesopores,while the PV is dominated by meso-and macropores.The pore structures are more complex and controlled by multiple factors,such as mineral compositions and diagenesis,but organic matter is not critical.The maximum absolute adsorption methane volume(VL)is 0.97−3.58 cm^(3)/g(mean 1.90 cm^(3)/g),correlating well with the total SSA,SSA,and pore volume of micropores,which indicates that methane is mainly adsorbed and stored in micropores,followed by mesopores.

A study on hydrocarbon generation potential of deep source rocks and gas sources in Changling Fault Depression,Songliao Basin

Through natural gas exploration in the Changling Fault Depression,abundant natural gas flows were obtained and high-quality source rocks were also reveiled.Based on bulk analyses,organic matter abundance,organic matter type and thermal maturity of the source rocks were studied systematically.At the same time,gas gener-ating intensity and gas generating quantity were quantitatively analyzed by using basin modeling technologies.The results indicated that gas source rocks of the Shahezi Formation are characterized by abundant organic matter and dark mudstone distributions and high-over thermal maturity.They possess high gas generating intensity with the biggest value surpassing 500×108 m3/km2,and large gas generating quantity,accounting from 74.40% of the total amount.The Yingcheng Formation has good gas-source rocks with moderate dark mudstone and relatively high organic matter.It has type-Ⅲ organic matter and low-over thermal maturity.The gas generating intensity is moderate,between 20×108 and 60×108 m3/km2.The gas generating quantity is 13.63% of the total amount.The Huoshiling Formation has relatively good source rocks with limited dark mudstone and low organic matter.It has type-Ⅲ kerogen and over-thermal maturity.The gas generating quantity is 7.10% of the total amount.The Denglouku Formation has poor-gas source rocks,and is characterized by undeveloped dark mudstone and low organic matter abundance.It has type-Ⅲ organic matter and low-high thermal maturity.Hydrocarbon gases in the Changling Fault Depression mainly originated from the Shahezi Formation,and secondarily from the Yingcheng Formation.The contribution of the Huoshiling Formation gas source rocks is relatively small.

Formation mechanism of high-quality reservoirs of Lower Cretaceous volcaniclastic sedimentary rocks in Songliao Basin

High abundance gas reservoirs were discovered in the volcaniclastic sedimentary rocks of Wangfu faulted depression in Songliao Basin,but the distribution pattern and formation mechanism of this type of high-quality reservoirs were not clear.In the Shahezi Formation of Well Block CS6 in Wangfu faulted depression,the void space type,reservoir property,pore throat and pore diameters were studied by means of the casting thin section,helium intrusion porosimetry,mercury intrusion porosimetry(MIP)and nuclear magnetic resonance(NMR)methods,moreover,formation mechanism and main controlling factors of the high-quality reservoirs were discussed.Results showed that lithology of high-quality reservoirs consisted of the volcaniclastic sedimentary agglomerate and the volcaniclastic sedimentary breccia,and exhibited the coarsening-upward characteristics.Furthermore,three types(seven subtypes)of void spaces were identified,among which the intragranular vesicles with primary abundant pores were uniquely developed in the high-quality reservoirs of volcaniclastic sedimentary rocks,indicating this high-quality reservoir belonged to the large-pore and small-throat reservoir;porosity and permeability of volcaniclastic sedimentary agglomerate were slightly higher than those of volcaniclastic sedimentary breccia,and both were significantly higher than those of sedimentary rock in the Shahezi Formation,the porosity was closely related with the burial depth.Formation mechanisms of reservoirs in the volcaniclastic sedimentary rocks were release of volatile matters in parent rocks,grain support,dissolution and tectonic modification,among which the former two were the main controlling factors.The high-quality reservoirs over the study area were mainly distributed in the central part of the fan of volcaniclastic sedimentary rocks,among which the grain-supported,coarse-grained volcaniclastic sedimentary rocks with high content of intragranular vesicular detritus and burial depth less than 3000 m were the most favorable area for development of the high-quality reservoirs.