Development potential and technical strategy of continental shale oil in China

Continental shale oil is a general term for liquid hydrocarbons and many kinds of organic matter in continental organic-rich shale series with vitrinite reflectance of more than 0.5%at buried depth of more than 300 m,and is an important type of source-rock oil and gas.Based on the evolution model of oil generation and expulsion in organic-rich shale series controlled by maturity,continental shale oil is divided into two types:medium-high maturity and medium-low maturity.(1)The continental shale series in China develop high-quality source rocks of freshwater and saltwater lacustrine facies,as well as multiple types of reservoirs,including clastic rocks,carbonate rocks,diamictite,tuff and shale,forming a number of"sweet sections"and"sweet areas"of continuous distribution inside or near source rocks,which have large scale resources.(2)Experimental analysis of organic rich shale samples shows that the shale samples with wavy and horizontal beddings have good storage conditions,and the horizontal permeability of shale is tens to hundreds of times of its vertical permeability,which is conducive to the lateral migration and accumulation of shale oil in the source rocks.(3)After evaluation,the geological resources of medium-high maturity shale oil are about 10 billion tons,which can be effectively developed by horizontal drilling and volumetric fracturing,and will be a practical field of oil exploration in recent years.Shale oil with medium and low maturity has huge resource potential,and technological recoverable resources of(70?90)billion tons,making it a strategic alternative resource of oil industry.However,economic development of this type of shale oil needs in-situ conversion technology breakthroughs.Continental shale oil is an inevitable choice in the process of Chinese continental petroleum exploration from"outside source"to"inside source".Making breakthroughs in the core technologies such as"sweet area"evaluation and optimization,horizontal well volume fracturing and in-situ conversion technology and equipment is the key to realizing scale development of continental shale oil economically.

“Component flow”conditions and its effects on enhancing production of continental medium-to-high maturity shale oil

Based on the production curves,changes in hydrocarbon composition and quantities over time,and production systems from key trial production wells in lacustrine shale oil areas in China,fine fraction cutting experiments and molecular dynamics numerical simulations were conducted to investigate the effects of changes in shale oil composition on macroscopic fluidity.The concept of“component flow”for shale oil was proposed,and the formation mechanism and conditions of component flow were discussed.The research reveals findings in four aspects.First,a miscible state of light,medium and heavy hydrocarbons form within micropores/nanopores of underground shale according to similarity and intermiscibility principles,which make components with poor fluidity suspended as molecular aggregates in light and medium hydrocarbon solvents,such as heavy hydrocarbons,thereby decreasing shale oil viscosity and enhancing fluidity and outflows.Second,small-molecule aromatic hydrocarbons act as carriers for component flow,and the higher the content of gaseous and light hydrocarbons,the more conducive it is to inhibit the formation of larger aggregates of heavy components such as resin and asphalt,thus increasing their plastic deformation ability and bringing about better component flow efficiency.Third,higher formation temperatures reduce the viscosity of heavy hydrocarbon components,such as wax,thereby improving their fluidity.Fourth,preservation conditions,formation energy,and production system play important roles in controlling the content of light hydrocarbon components,outflow rate,and forming stable“component flow”,which are crucial factors for the optimal compatibility and maximum flow rate of multi-component hydrocarbons in shale oil.The component flow of underground shale oil is significant for improving single-well production and the cumulative ultimate recovery of shale oil.

“Exploring petroleum inside source kitchen”: Shale oil and gas in Sichuan Basin

The Sichuan Basin is rich in shale oil and gas resources,with favorable geological conditions that the other shale reservoirs in China cannot match.Thus,the basin is an ideal option for fully"exploring petroleum inside source kitchen"with respect to onshore shale oil and gas in China.This paper analyzes the characteristics of shale oil and gas resources in the United States and China,and points out that maturity plays an important role in controlling shale oil and gas composition.US shale oil and gas exhibit high proportions of light hydrocarbon and wet gas,whereas Chinese marine and transitional shale gas is mainly dry gas and continental shale oil is generally heavy.A comprehensive geological study of shale oil and gas in the Sichuan Basin reveals findings with respect to the following three aspects.First,there are multiple sets of organic-rich shale reservoirs of three types in the basin,such as the Cambrian Qiongzhusi Formation and Ordovician Wufeng Formation-Silurian Longmaxi Formation marine shale,Permian Longtan Formation transitional shale,Triassic Xujiahe Formation lake-swamp shale,and Jurassic lacustrine shale.Marine shale gas enrichment is mainly controlled by four elements:Deep-water shelf facies,moderate thermal evolution,calcium-rich and silicon-rich rock association,and closed roof/floor.Second,the"sweet section"is generally characterized by high total organic carbon,high gas content,large porosity,high brittle minerals content,high formation pressure,and the presence of lamellation/bedding and natural microfractures.Moreover,the"sweet area"is generally characterized by very thick organic-rich shale,moderate thermal evolution,good preservation conditions,and shallow burial depth,which are exemplified by the shale oil and gas in the Wufeng-Longmaxi Formation,Longtan Formation,and Daanzhai Member of the Ziliujing Formation.Third,the marine,transitional,and continental shale oil and gas resources in the Sichuan Basin account for 50%,25%,and 30%of the respective types of shale oil and gas geological resources in China,with great potential to become the cradle of the shale oil and gas industrial revolution in China.Following the"Conventional Daqing-Oil"(i.e.,the Daqing oilfield in the Songliao Basin)and the"Western Daqing-Oil&Gas"(i.e.,the Changqing oilfield in the Ordos Basin),the Southwest oil and gas field in the Sichuan Basin is expected to be built into a"Sichuan-Chongqing Daqing-Gas"in China.