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The discovery and large-scale exploration of unconventional oil/gas resources since 1980s have been considered as the most important advancement in the history of petroleum geology; that has not only changed the balance of supply and demand in the global energy market, but also improved our understanding of the formation mechanisms and distribution characteristics of oil/gas reservoirs. However, what is the difference of conventional and unconventional resources and why they always related to each other in petroliferous basins is not clear. As the differences and correlations between unconventional and conventional resources are complex challenging issues and very critical for resources assessment and hydrocarbon exploration, this paper focused on studying the relationship of formations and distributions among different oil/gas reservoirs. Drilling results of 12,237 exploratory wells in 6 representative petroliferous basins of China and distribution characteristics for 52,926 oil/gas accumulations over the world were applied to clarify the formation conditions and genetic relations of different oil/gas reservoirs in a petroliferous basin, and then to establish a unified model to address the differences and correlations of conventional and unconventional reservoirs. In this model, conventional reservoirs formed in free hydrocarbon dynamic field with high porosity and permeability located above the boundary of hydrocarbon buoyancy-driven accumulation depth limit. Unconventional tight reservoirs formed in confined hydrocarbon dynamic field with low porosity and permeability located between hydrocarbon buoyancy-driven accumulation depth limit and hydrocarbon accumulation depth limit. Shale oil/gas reservoirs formed in the bound hydrocarbon dynamic field with low porosity and ultra-low permeability within the source rock layers. More than 75% of proved reserves around the world are discovered in the free hydrocarbon dynamic field, which is estimated to contain only 10% of originally generated hydrocarbons. Most of undiscovered resources distributed in the confined hydrocarbon dynamic field and the bound hydrocarbon dynamic field, which contains 90% of original generated hydrocarbons, implying a reasonable and promising area for future hydrocarbon explorations. The buried depths of hydrocarbon dynamic fields become shallow with the increase of heat flow, and the remaining oil/gas resources mainly exist in the deep area of “cold basin” with low geothermal gradient. Lithology changing in the hydrocarbon dynamic field causes local anomalies in the oil/gas dynamic mechanism, leading to the local formation of unconventional hydrocarbon reservoirs in the free hydrocarbon dynamic field or the occurrence of oil/gas enrichment sweet points with high porosity and permeability in the confined hydrocarbon dynamic field. The tectonic movements destroy the medium conditions and oil/gas components, which leads to the transformation of conventional oil/gas reservoirs formed in free hydrocarbon dynamic field to unconventional ones or unconventional ones formed in confined and bound hydrocarbon dynamic fields to conventional ones.

Great changes of the global energy industry have been caused by the rapid development of unconventional oil and gas. It is necessary to deeply consider the profound influence of the unconventional oil and gas revolution on the classical petroleum geology theory and to review geological conception of oil and gas accumulation elements and theoretic framework of petroleum system, giving the petroleum geology a new academic connotation. The author summarizes the significant progresses of global unconventional oil and gas exploration and development, and points out that the unconventional oil and gas revolution not only has a significant economic significance of oil and gas resource increment, but also brings great innovation to the theory of petroleum geology, thus having important scientific significances. This paper summarizes the core contents of four aspects of hydrocarbon generation, reservoir, distribution and development in classical petroleum geology, and comprehensively reviews the five important nodes in the developmental history of petroleum geology, which include anticline and trap theory, hydrocarbon generation from organic matter and petroleum system theory, continental petroleum geology, marine deepwater petroleum geology, continuous hydrocarbon accumulation and unconventional petroleum geology theory. Unconventional oil and gas has made a great breakthrough to classical petroleum geology on the basic theoretical concepts such as trap, reservoir, caprock, resource distribution, and enrichment, thereby promoting the basic research on petroleum geology to transform into the whole process of hydrocarbon generation, whole type of reservoir, and whole genetic mechanism, deepening unconventional petroleum geology theory, promoting the development and reconstruction of petroleum geology system, representing great significances to the strategic development from conventional to unconventional oil and gas in China or even in the world. Key words: petroleum geology, unconventional petroleum geology theory, continuous hydrocarbon accumulation, tight oil and tight gas, shale oil and shale gas, oil and gas production

As petroleum exploration advances and as most of the oil–gas reservoirs in shallow layers have been explored, petroleum exploration starts to move toward deep basins, which has become an inevitable choice. In this paper, the petroleum geology features and research progress on oil–gas reservoirs in deep petroliferous basins across the world are characterized by using the latest results of worldwide deep petroleum exploration. Research has demonstrated that the deep petroleum shows ten major geological features. (1) While oil–gas reservoirs have been discovered in many different types of deep petroliferous basins, most have been discovered in low heat flux deep basins. (2) Many types of petroliferous traps are developed in deep basins, and tight oil–gas reservoirs in deep basin traps are arousing increasing attention. (3) Deep petroleum normally has more natural gas than liquid oil, and the natural gas ratio increases with the burial depth. (4) The residual organic matter in deep source rocks reduces but the hydrocarbon expulsion rate and efficiency increase with the burial depth. (5) There are many types of rocks in deep hydrocarbon reservoirs, and most are clastic rocks and carbonates. (6) The age of deep hydrocarbon reservoirs is widely different, but those recently discovered are predominantly Paleogene and Upper Paleozoic. (7) The porosity and permeability of deep hydrocarbon reservoirs differ widely, but they vary in a regular way with lithology and burial depth. (8) The temperatures of deep oil–gas reservoirs are widely different, but they typically vary with the burial depth and basin geothermal gradient. (9) The pressures of deep oil–gas reservoirs differ significantly, but they typically vary with burial depth, genesis, and evolution period. (10) Deep oil–gas reservoirs may exist with or without a cap, and those without a cap are typically of unconventional genesis. Over the past decade, six major steps have been made in the understanding of deep hydrocarbon reservoir formation. (1) Deep petroleum in petroliferous basins has multiple sources and many different genetic mechanisms. (2) There are high-porosity, high-permeability reservoirs in deep basins, the formation of which is associated with tectonic events and subsurface fluid movement. (3) Capillary pressure differences inside and outside the target reservoir are the principal driving force of hydrocarbon enrichment in deep basins. (4) There are three dynamic boundaries for deep oil–gas reservoirs; a buoyancy-controlled threshold, hydrocarbon accumulation limits, and the upper limit of hydrocarbon generation. (5) The formation and distribution of deep hydrocarbon reservoirs are controlled by free, limited, and bound fluid dynamic fields. And (6) tight conventional, tight deep, tight superimposed, and related reconstructed hydrocarbon reservoirs formed in deep-limited fluid dynamic fields have great resource potential and vast scope for exploration. Compared with middle–shallow strata, the petroleum geology and accumulation in deep basins are more complex, which overlap the feature of basin evolution in different stages. We recommend that further study should pay more attention to four aspects: (1) identification of deep petroleum sources and evaluation of their relative contributions; (2) preservation conditions and genetic mechanisms of deep high-quality reservoirs with high permeability and high porosity; (3) facies feature and transformation of deep petroleum and their potential distribution; and (4) economic feasibility evaluation of deep tight petroleum exploration and development.

The Kuqa foreland between the Tarim basin and the Tianshan Mountains is rich in oil and gas. Based on field work and seismic profiles, the structural styles and their formation mechanisms were determined, and the role of evaporites in the deformation was demonstrated. The main structural styles in the overburden are detachment folds, large scale nappes, triangle zones, gentle and wide synclines, fault-propagation folds and pop-ups. The main structures in the substrate are small-scale thrust faults, duplexes, pop-ups and fault-bend and fault-propagation folds, and formed mainly at the end of the Pliocene under north-south compression. The evaporite layer in the lower section of the Paleogene is the decollement zone for the disharmonic deformations in the overburden and in the substrate. The detachment along the evaporite layer made it possible for compressive stresses to be transmitted farther in the overburden than in the substrate. Deformation in the overburden is more extensive than in the substrate at the leading edge of deformation. At the trailing edge of deformation, the structural highs in the overburden closely correspond to those in the substrate, which is of significance for petroleum exploration in the western Kuqa foreland.

Thick salt beds in the Lower Tertiary were developed in the Kuqa foreland fold belt of the northern Tarim Basin. The salt beds controlled the structural deformation and hydrocarbon accumulation of the belts. Because of the salt beds, the Meso-Cenozoic structures of the Kuqa foreland fold belts can be divided into three major tectono-sequences: suprasalt structures, salt bed structures and subsalt structures. Suprasalt structures include thrusts and fault-related folds. Salt bed structures are dominated by salt pillows, allochthonous salt sheets, salt welds and fish-tail structures. Subsalt structures mainly include imbricated thrust belts, duplex structures, fault-related folds and pop-up structures. Formation of the salt structures in the Kuqa foreland fold belts is controlled by compression, gravitational gliding and gravitational spreading. The basin-mountain coupling and compression resulted in intense shortening of the fold belt. The rising of the South Tianshan Mountains caused the gravity gliding and gravity spreading. Plastic flow deformation of the salt beds resulted in salt thickening from the mountain front to the south of the fold belt. The thick salt bed accumulated in the Klasu and Qiulitag structural belts, and controlled the distribution of the salt-related structural traps and oil–gas pools. The structural traps have a bearing upon salt beds in the Kuqa foreland fold belts. The source rocks are mainly located below the salt beds, which can act as excellent cap rocks and seal the gas pools. The subsalt beds are the favorable places for hydrocarbons to accumulate.

There is a gas-rich and well-charged petroleum system in the Kuqa Depression where Triassic and Jurassic source rocks play important roles. Distributed in an area of more than 10000 km and with a thickness of up to 1000 m, they are composed of dark mudstones, carbonaceous mudstones and coal seams containing 6%, 40% and 90% of TOC, respectively, and are mainly the humic organic matter. As high-quality regional cap rocks, the Neogene and Eogene gypsum rocks and gypseous mudstones matched well with the underlying Neogene and Cretaceous-Eogene sandstones. They have formed the most favorable reservoir-seal assemblages in the Kuqa Depression. Also the Jurassic sandstones and mudstones formed another favorable reservoir-seal assemblage. The traps are shaped late in the fold-thrust belt, mainly fixed in the Tertiary-Quaternary, where ten structure styles have been distinguished. These traps spread as a zone in N-S, are scattered like a segmental line in W-E and show tier-styled vertically. The best traps are gypsum-salt covered fault-bend anticlines related to the passive roof duplex. This petroleum system is characterized by late accumulation. In the early Himalayan Movement, mainly gas condensate and oil accumulated and were distributed in the outer circular region of the kitchen; whereas in the middle and late Himalayan the gas accumulations mainly formed and were distributed in the inner circular region near the kitchen. The overpressure of gas pools is common and is formed by seal capacity of thick gypsum layers, extensive tectonic compression and large uplift. The well-preserved anticline traps underlying the high-quality regional cap rocks of the Tertiary gypsum rocks and gypseous mudstones are the main targets for the discovery of giant and medium-sized gas fields. Above conclusions are important for the petroleum geology theory and the exploration of the fold-thrust belt in foreland basins in central and western China.

Since the 1950’s, along with the discoveries of the Daqing oil field and exploration in the Bohai bay area, terrestrial petroleum geology exploration has been gradually developed. The theory of oil generation in a terrestrial basin and multiple oil-gas accumulation zones is the core of terrestrial petroleum geology. This new hypothesis not only improved the theory of petroleum geology, but it also effectively directed oil and gas exploration in the Chinese terrigenous basin. In addition, numerous structural types of oil and gas reservoirs were discovered, more than a dozen oil and gas production bases were developed, which include Daqing, Bohai bay, Changqing and Xinjiang, and which effectively supported the construction and development of the Chinese national economy.

Carbonate rocks are important for oil and gas reservoir and petroleum production globally; the oil and gas production from carbonate rocks represented 60% of total production worldwide (Roeh and Choquette, 1985). In China, carbonate rocks occur in an area of 250 ×104 km2 that represent 55% of the total area covered by sedimentary rocks. Carbonate rocks mainly occur in the Sichuan basin, the Tarim basin and the Eerduosi basin (Fu et al., 2002; Zhao et al., 2002; Zhao 2004). In China, the total thickness of stratigraphic sequences from the late Proterozoic eonothem to the Cenozoic erathem is approximately 27,000 m; the thickness of the carbonate rocks is approximately 11,000 m, which represents 41% of the total stratigraphic thickness in China (Zou et al., 2007; Lu et al., 2000; Dai et al., 1997; Dai 1999; Dai 2003; Zhang et al., 2003; Lou et al., 2001; Dou et al., 2003). Most of these are marine scarbonate rocks and the lacustrine carbonate rocks only appear in the Mesozoic-Cenozoic strata.

The Chinese petroleum industry is famous for producing oil and gas from terrigenous sedimentary basins. As a result, geological studies of the terrigenous petroleum basin and the use of related technologies for petroleum exploration and development are at an advanced stage in the world. If the idea that terrigenous stratigraphy can generate oil is the central idea behind the geological theory, then the characteristics of reservoir sequences in the terrigenous basins are the significant foundation of this geological theory. This chapter will discuss the distribution of the stratigraphy, the major types of depositional facies and the characteristics of the petroleum system in the Chinese petroleum basins. In addition, the nature of the depositional evolution in the major petroleum basins will be briefly introduced.