Geological conditions and reservoir characteristics of various shales in major shalehosted regions of China

China is home to shales of three facies:Marine shale,continental shale,and marine-continental transitional shale.Different types of shale gas are associated with significantly different formation conditions and major controlling factors.This study compared the geological characteristics of various shales and analyzed the influences of different parameters on the formation and accumulation of shale gas.In general,shales in China’s several regions exhibit high total organic carbon(TOC)contents,which lays a sound material basis for shale gas generation.Marine strata generally show high degrees of thermal evolution.In contrast,continental shales manifest low degrees of thermal evolution,necessitating focusing on areas with relatively high degrees of thermal evolution in the process of shale gas surveys for these shales.The shales of the Wufeng and Silurian formations constitute the most favorable shale gas reservoirs since they exhibit the highest porosity among the three types of shales.These shales are followed by those in the Niutitang and Longtan formations.In contrast,the shales of the Doushantuo,Yanchang,and Qingshankou formations manifest low porosities.Furthermore,the shales of the Wufeng and Longmaxi formations exhibit high brittle mineral contents.Despite a low siliceous mineral content,the shales of the Doushantuo Formation feature a high carbonate mineral content,which can increase the shales’brittleness to some extent.For marine-continental transitional shales,where thin interbeds of tight sandstone with unequal thicknesses are generally found,it is recommended that fracturing combined with drainage of multiple sets of lithologic strata should be employed to enhance their shale gas production.

Geological Conditions and Prospect Forecast of Shale Gas Formation in Qiangtang Basin, Qinghai-Tibet Plateau

The presence of shale gas has been confirmed in almost every marine shale distribution area in North America.Formation conditions of shale gas in China are the most favorable for marine,organic-rich shale as well.But there has been little research focusing on shale gas in Qiangtang Basin,Qinghai-Tibet Plateau,where a lot of Mesozoic marine shale formations developed.Based on the survey results of petroleum geology and comprehensive test analysis data for Qinghai-Tibet Plateau,for the first time,this paper discusses characteristics of sedimentary development,thickness distribution,geochemistry,reservoir and burial depth of organic-rich shale,and geological conditions for shale gas formation in Qiangtang Basin.There are four sets of marine shale strata in Qiangtang Basin including Upper Triassic Xiaochaka Formation （T3x）,Middle Jurassic Buqu Formation （J2b）,Xiali Formation （J2x） and Upper Jurassic Suowa Formation （J3s）,the sedimentary types of which are mainly bathyal-basin facies,open platform-platform margin slope facies,lagoon and tidal-fiat facies,as well as delta facies.By comparing it with the indicators of gas shale in the main U.S.basins,it was found that the four marine shale formations in Qiangtang Basin constitute a multi-layer distribution of organic-rich shale,featuring a high degree of thickness and low abundance of organic matter,high thermal evolution maturity,many kinds of brittle minerals,an equivalent content of quartz and clay minerals,a high content of feldspar and low porosity,which provide basic conditions for an accumulation of shale gas resources.Xiaochaka Formation shale is widely distributed,with big thickness and the best gas generating indicators.It is the main gas source layer.Xiali Formation shale is of intermediate thickness and coverage area,with relatively good gas generating indicators and moderate gas formation potential.Buqu Formation shale and Suowa Formation shale are of relatively large thickness,and covering a small area,with poor gas generating indicators,and limited gas formation potential.The shale gas geological resources and technically recoverable resources were estimated by using geologic analogy method,and the prospective areas and potentially favorable areas for Mesozoic marine shale gas in Qiangtang Basin are forecast and analyzed.It is relatively favorable in a tectonic setting and indication of oil and gas,shale maturity,sedimentary thickness and gypsum-salt beds,and in terms of mineral association for shale gas accumulation.But the challenge lies in overcoming the harsh natural conditions which contributes to great difficulties in ground engineering and exploration,and high exploration costs.

Geological conditions of natural gas accumulation and new exploration areas in the Mesoproterozoic to Lower Paleozoic of Ordos Basin, NW China

Based on field outcrop investigation,interpretation and analysis of drilling and seismic data,and consulting on a large number of previous research results,the characteristics of ancient marine hydrocarbon source rocks,favorable reservoir facies belts,hydrocarbon migration direction and reservoir-forming law in the Ordos Basin have been studied from the viewpoints of North China Craton breakup and Qilian-Qinling oceanic basin opening and closing.Four main results are obtained:(1)Controlled by deep-water shelf-rift,there are three suites of source rocks in the Ordos Basin and its periphery:Mesoproterozoic,Lower Cambrian and Middle-Upper Ordovician.(2)Controlled by littoral environment,paleo-uplift and platform margin,four types of reservoirs are developed in the area:Mesoproterozoic-Lower Cambrian littoral shallow sea quartz sandstone,Middle-Upper Cambrian–Ordovician weathering crust and dolomitized reservoir,and Ordovician L-shape platform margin reef and beach bodies.(3)Reservoir-forming assemblages vary greatly in the study area,with"upper generation and lower storage"as the main pattern in the platform,followed by"self-generation and self-storage".There are both"upper generation and lower storage"and"self-generation and self-storage"in the platform margin zone.In addition,in the case of communication between deep-large faults and the Changchengian system paleo-rift trough,there may also exist a"lower generation and upper reservoir"combination between the platform and the margin.(4)There are four new exploration fields including Qingyang paleo-uplift pre-Carboniferous weathering crust,L-shape platform margin zone in southwestern margin of the basin,Ordovician subsalt assemblage in central and eastern parts of the basin,and Mesoproterozoic–Cambrian.Among them,pre-Carboniferous weathering crust and L-shape platform margin facies zone are more realistic replacement areas,and Ordovician subsalt assemblage and the Proterozoic-Cambrian have certain potential and are worth exploring.

Geological and Geophysical Conditions for Potential Natural Gas Hydrate Resources in Southern South China Sea Waters

The water depth in Nansha（南沙） waters,which is located in the southern South China Sea,varies from 200 to 2 500 m,with a deep-water（500 m） area of 500 000 km2.In this region,there are many depositional basins with various structural features,prone to the accumulation of organic material.The temperature and pressure conditions in the deep-water environment are suitable for the preservation of gas hydrate.At several sites,we have recognized bottom-simulating reflectors（BSRs） from seismic data.Regional geology analyses show that the Nansha waters may have abundant gas hy-drate prospects,especially in localities such as the Nansha trough and other deep-water basins of the central Nansha waters.

Geological conditions of coal and gas(oil)-bearingbasins in China

There are various types of coal basins in China. Indosinian movement can be re- garded as their evolutionary limit, and the basins can be divided into two developmental stages, three structural patterns and two sedimentary environments. However, only those coal measure strata that have been deeply buried in the earth are possible to be converted into coal and gas (oil)-bearing basins. Among which, only part of the coal measures possess the essential geo- logical conditions to the formation of commercial humic oil. However, humic gas will be the major exploration target for natural gas in China. Among various coal basins, foreland basins have the best prospect for humic gas. Rift (faulted) basins accumulate the most abundance of humic gas, and are most favorable to generate humic oil. Craton basins have relatively low abundance of humic gas, but the evolution is rather great. The three kinds of coal basins mentioned above constitute China’s three primary accumulation areas of humic gas: western, central and offshore areas. The major basins for humic gas field exploration include Tarim, Ordos, Sichuan, East China Sea and Yingqiong basins.

Geological characteristics of large gas provinces and large gas fields in China

Based on the examination of the global researches on oil and gas provinces and large gas fields and the analysis of the features,attributes and distribution of large gas provinces and gas fields,this paper puts forward three indicators of determining large oil and gas provinces:spatial indicator,reservoir-forming indicator and resource indicator.It classifies the gas accumulated areas and large gas provinces in China and analyzes the controlling factors on the distribution of large gas provinces and large gas fields:the lateral distribution is mainly controlled by high-energy sedimentary facies and constructive diagenetic facies,palaeo-highs and their periclinal zones,deep faults,etc,and the vertical distribution is mainly controlled by unconformities,series of evaporates and deep low-velocity highly-conductive beds,etc.It also reveals the main geological characteristics of large gas provinces and large gas fields in China.Large gas fields in four-type basins have their own characteristics and onland large gas fields are dominantly developed in foreland basins and craton basins;there are three types of gas sources,of which,coal is the main source with high gas generating intensity and varying origins;reservoir rocks of the large gas fields(provinces)are of various types and dominated generally by low-middle permeability and porosity pore-type reservoirs;structural traps and litho-stratigraphic traps coexist in Chinese large gas fields and form dense high abundance and large-area low and middle-abundance large gas fields;most of the large gas fields have late hydrocarbon-generation peaks and reservoir formation,and experienced the process of multiple-stage charging and late finalization; large gas provinces(fields)have good sealing and preservation conditions,and evaporates seals are largely developed in large and extra-large gas fields.This paper intends to shed light on the exploration and development of large gas fields(provinces)through analyzing their geological characteristics.

The generation and its sealing condition of natural gas in the Tadong area

Focusing on the two natural gas exploration geological problems with abundant source of oil cracking gas in the late stage and the sealing condition of the oil cracking gas reservoir,the kinetics of oil cracking gas and the evaluation parameters of gas cap rock are adopted to the study on the natural gas accumulation conditions in the Tadong area. Both the study on the kinetics of oil cracking gas and the statistical results of reservoir bitumen reveal that the geological formation of oil cracking gas in the Tadong area is located in the top of Cambrian. Two kinds of oil cracking gas geological models at least,namely well Mandong-1's early rapid generation model(Middle Ordovician-end Silurian) and peak cracking model(with the natural gas conversion rate >90%) ,namely well Yingnan-2's two-stage generation model of oil cracking gas,have been set up. The oil cracking gas of Yingnan-2 in the late stage is very significant in the evaluation of natural gas exploration in the Tadong area. The evaluation results of the cap rock show that the microscopic parameters of cap rock from the lower assemblage of Cambrian-Ordovician are better than those from the upper assemblage. The former has strong capillary sealing ability and higher cap rock breakthrough pressure than the upper assemblage,with strong sealing ability,so that natural gas dissipates mainly by diffusion. According to the above investigations,the lower assemblage Cambrian-Ordovician natural gas of Kongquehe slope,Tadong low uplift and Yingjisu depression in the Tadong area prospects well.