Hydrocarbon source rocks are characterized by the hydrocarbon discharge, and the alteration and variation in organic compositions and organic content due to the enhanced thermal maturation. These variations throw constraints on the application of the conventional inversion evaluation of hydrocarbon potential by assessing the residual organic matter left in source rocks. Geobiology, probing the interaction between the life system and the earth system, provides new principles in deciphering the whole dynamic processes related to the organic evolution history from living biomass to organic burial. Geobiological subdisciplines, including molecular geobiology, geomicrobiology, geoecology and biogeochemistry, offer new methodology and techniques to estimate the paleoproductivity, depositional organics and organic burial capacity and their components. Geobiofacies, newly proposed herein, is terminologized to define the geobiological dynamic processes through the combination of biofacies with organic facies and sedimentary facies, and expressed by the biohabitat types, paleoproductivity, depositional and preserved organics. Geobiofacies is identified as a useful means to create the geobiological evaluation system, which in turn rectifies the conventional evaluation system for the marine source rocks.

Understanding the dynamics of organic matter in modern marine water columns greatly favors the geobiologcal evaluation of hydrocarbon source rocks. Biolipids could make great contribution to petroleum hydrocarbons due to their comparable chemical components and the slightly refractory characteristics of biolipids during the microbial/thermal degradation. A variety of environmental factors such as temperature, CO₂ and salinity could affect the biochemical contents in microorganisms. As a result, microorganisms living in a changing environmental condition might have a different contribution to the petroleum formation. Organic carbon flux is shown to bear a positive correlation with the primary productivity only within a certain range of biomass volumes in a specific biohabitat. Furthermore, organic matter is degraded much quickly in a water column with oxic conditions. Therefore, the anoxic condition, along with the enhanced biological productivity, would be one of the significant factors in the formation of high-quality hydrocarbon source rocks. The formation of biofilms and microbial mats favors the preservation of sedimentary organic matter by decreasing the degradation rate of organic matter. Identification of biofilms and microbial mats in sedimentary rocks will thus greatly help to understand the depositional processes of organic matter finally preserved in hydrocarbon source rocks.

In this paper, the mean primary productivity of the Talung Formation in the deep-water basin (50–500 m depth) of Dongpan, Guangxi, South China, was calculated, roughly ranging from 0.1 to 0.6 g C · m^-2 · day^-1 on the basis of the content of the trace element Cu. Results showed that the primary productivity obtained was comparable with the previously-reported data for the black shale of the Phosphoria Formation, a Permian phosphate deposit in the northwest United States, and also similar to that of the modern deposit in the Cariaco basin, Venezuela. It was observed that the primary productivity increased with the enhanced abundance of algae and photosynthetic bacteria, and with the decrease in both the radiolarian individuals and the body size of brachiopods during the faunal mass extinction. This ecological coupling indicates that the changes of primary productivity are closely related to the propagation and the decline of producers (algae and bacteria), and that consumers (radiolarians and brachiopods) probably have little influence on the changes of primary productivity.

A quantitative reconstruction method of paleo-primary productivity is present on the basis of the investigation on the faunal record in cherts. By obtaining the accumulation rate of radiolarian shells in the cherts and considering all aspects of silica loss before burial, such as dissolution in the eutrophic zone or within the sediments, it is possible to reconstruct the production rate of radiolarian in the surface water, which would be used to estimate the paleo-primary productivity. This method is applied to the end Permian chert sequence from Dalong Formation in Dongpan area, southwestern China. The resulting paleo-primary productivity of the ocean in end Permian south China is about 477.9 g C/m²/a during the sedimentation of cherts, which, to some extent, agrees with the primary productivity reported for some modern oceans.

The paper presents the molybdenum isotope data, along with the trace element content, to investigate the geochemical behavior of authigenic Mo during long-term burial in sediments in continental margin settings of the Yangtze block, as well as their indication to the burial of original organic carbon. The burial rate of original organic carbon was estimated on the basis of the amount of sedimentary sulfur (TS content), whilst the carbon loss by aerobic degradation was estimated according to calculated Mn contents. On these points, the original organic carbon flux was calculated, exhibiting a large range of variation (0.17–0.67 mmol/m²/day). The strong correlation between sedimentary Mo isotope values and organic carbon burial rates previously proposed on the basis of the investigations on modern ocean sediments, was also used here to estimate the organic carbon burial rate. The data gained through this model showed that organic carbon burial rates have large variations, ranging from 0.43–2.87 mmol/m²/day. Although the two sets of data gained through different geochemical records in the Yangtze block show a deviation of one order of magnitude, they do display a strong correlation. It is thus tempting to speculate that the Mo isotope signature of sediments may serve as a tracer for the accumulation rate of original organic carbon in the continental margin sediments.

By combining the carbon cycle model with the records of carbonate and organic (kerogen) carbon isotope, this paper presents the calculation of the fraction of organic carbon burial (f_org) of beds 23–40 at the global boundary stratotype section and point (GSSP) of the Permian–Triassic boundary at Meishan, Zhejiang Province. The resulting calculation produces two episodes of f_org maxima observed to occur at beds 23–24 and 27–29, which respectively corresponds to the two episodic anoxic events indicated by the flourish of green sulfur bacteria. Two episodic f_org minima occurred at beds 25–26 and 32–34, generally coincident with the flourish of cyanobacteria (bed 26 and upper part of beds 29 to 34) as shown by the high value of 2-melthyhopnoanes. It appears that the f_org is related to the redox conditions, with greater f_org values observed under the reductive condition. The relationship between f_org and the total organic carbon (TOC) content was complex. The f_org value was low at some beds with a high TOC content (such as bed 26), while high observed at some beds with a low TOC content (e.g. bed 27). This association infers the important contribution of primary productivity to the TOC content. The original organic burial could be thus calculated through the configuration of the function of the primary productivity and f_org, which can be used to correct the residual TOC measured today. This investigation indicates that compiling the organic-inorganic carbon isotopes with the carbon cycle model favors to understand the fraction of organic carbon burial, providing information for the reconstruction of the coupling among biota, environments and organic burial.

To decipher the origin of oxygen-deficient shelfal deposits is significant for tracing the distribution of marine source rocks and interpreting the evolution of depositional environment. The origin of the Middle Permian Chihsia Formation in South China remains a puzzle for long with its evident oxygen-deficient features but diverse benthos. This paper shows a typical Chihsian depositional rhythm composed of the massive and the laminated limestones with ecological and geochemical features. Massive bioclastic limestone from the rhythm was aerobic in paleoxygenation condition indicated by both the ecological and geochemical features. However, a contradictory oxygenation was inferred for the laminated  counterpart from the rhythm, with the ecological signal being aerobic and the geochemical one being anoxic. The difference in ecological and geochemical indications was interpreted as the instability of paleoxygenation condition in shelf environments, caused by an enhanced paleoproductivity. Rhythmic occurrence of the oxygen-deficient condition might have been stemmed from paleo-Tethyan paleocurrents flowing across South China.

The calci-mircobialite is a special carbonate buildup, which is formed due to the activities of different kinds of microbes. Abundant microfossils preserved in the microbialite show the high-level productivity during deposition, while characteristic sedimentary minerals and geochemical compositions suggest an anoxic marine environment for organic burial. The high-level productivity and anoxic sedimentary environment favor the efficient preservation of organic matter and thus the formation of source rocks. On these points, microbialites could be one of the potential hydrocarbon source rocks, awaiting further geobiological investigation and exploration. Precambrian and some of the great transitional stages in Phanerozoic are critical periods when mircobialites were well developed. Widespread microbialites have been found in North and South China. Bitumen observed in many outcrops of Precambrian and late Devonian mircobialites further raises the possibility of the calci-microbialite as a potential hydrocarbon source rock.

To understand the potential mechanism of marine extremophiles participating in the formation and the evo lution of hydrocarbon resources in marine extreme environments, some typical kinds of extremophiles and their distributions in marine hydrothermal and cold vents are discussed and evaluated respectively. The potential relationship between extremophile activities and hydrocarbon resources in marine extreme environments are then discussed in details. It could be now preliminary concluded that archaea and bacteria are the two main kinds of extremophiles in marine extreme environments. The dominating microbe com munities in hydrothermal vents are heterotrophic zymogens, sulfate reducers and methanogens, while the ANME-2 group (Methanosarcinales) surrounded by sulfate-reducing bacteria and ANME-1 group dominate in cold vents. Marine extremophiles would be able to use CH₄ and H₂S to synthesize energy for metabolism and to support food chains for other unique macrobiota nearby, which together present a high abundance but a low diversity with distinct characteristics of horizontal and vertical distributions. Marine extremophiles might play an important role either directly or indirectly in the processes of hydrocarbon formation and subsequent alteration, and could indicate the evolution of hydrocarbon resources in marine extreme environments. Our research thus has a great significance both in theoretical approach of potential hydrocarbon resources formed by marine extremophile activities and in practical exploration of the potential hydrocarbon-source sedimentary layers formed in the Earth history or the potential strata in southern China.

The Late Permian Dalong Formation in Hubei, Hunan, Guizhou and Guangxi in South China could be subdivided into several sedimentary types according to the lithology, indicative of the great spatial variations of both the lithology and sedimentary facies. The Dalong Formation deposited during the highest transgression of Late Permian, in particular under the anaerobic and suboxic conditions, which shows a strong linkage with the hydrocarbon source rocks. The Dalong Formation has some favorable properties such as hydrocarbon source rocks, including the sediments deposited in highest transgression sea level, the deep slope sedimentary environments and the low sedimentary rate, which raises the importance of the investigations on the sedimentary features, the geochemical properties and their relationship with the oil potential.

Abundant normal alkanes, isoprenoid hydrocarbons, terpanes and steranes were detected in 23 samples taken from the carbonate platform to basin facies in the Devonian Frasnian–Famennian (F–F) transition of Guangxi, South China. They were mainly derived from the phytoplankton or bacteria and algae. Molecular stratigraphic parameters, abundant micron gypsums and pyrite framboids show that hypersaline and anoxia prevailed in the F–F transition, the highly stressed environments occurred near the F–F boundary. Orbital cyclostratigraphic studies indicate that the hypersaline and anoxia prevailed in the F–F transition spanned an interval at least from the Palmatolepis linguiformis conodont Zone to the Middle Palmatolepis triangularis conodont Zone and lasted about 1.2 Ma. The Devonian marine ecosystem underwent Early Devonian gradual optimization, Middle Devonian to the F–F transitional stepwise deterioration and late Famennian recovery. A positive feedback among higher surface ocean temperatures, hypersaline, water column anoxia, enhanced nutrient regeneration, phytoplankton productivity and eutrophication was probably substantial in the F–F transition in Guangxi, South China, even in other regions of the world, such as Holy Cross Mountains of Poland, Montagne Noire in southern France, Alberta in Canada. The point that should be emphasized is that the decisive and direct causes of the F–F transitional mass extinction would be the stepwise deterioration of the marine ecosystem since the Middle Devonian other than a bolide impact on the earth. The hypersaline and anoxia prevailed in the F–F transition favor the formation and preservation of hydrocarbon source rocks in South China.

It is known that high-quality, black-shale source rocks occur in the uppermost Ordovician Wufeng Formation and in the lowermost Silurian Longmaxi Formation in South China. Hence, it is important to understand their lithostratigraphy and the controls on their deposition. A review of lithostratigraphic criteria for subdividing the two adjacent formations provides new regional correlations between the formations and related stratigraphic successions and facies. Both the black shales and the related, overlying flysch deposits at the Ordovician–Silurian transition in South China appear to have migrated northwestward in time and space, reflecting probable flexural control in a foreland basin that developed in response to subduction-type orogeny southeast of the Yangtze block. The black shales also contain K-bentonites from explosive, felsic-intermediate volcanism, the distribution of which also supports orogeny to the southeast. Finally, the analysis of sequence stratigraphy, which shows that the initiation of transgressive system tracts (TST) and condensed section (CS) in the related third-order sequences coincided with the two black-shale horizons respectively, indicates that the main controlling factors for the deposition of the Ordovician–Silurian black shales in South China are (1) northwestwardly migrating, foreland-basin subsidence caused by deformational loading related to episodic accretion of the Cathaysia block to the Yangtze block during this period, and (2) the anoxic, sediment-starved water column caused by rapid rise of the sea-level during the two successive phases of third-order global sea-level rise near the Ordovician–Silurian transition in South China. In future exploration for hydrocarbon source rocks in the area, it is important to consider likely flexural and eustatic causes for subsiding, deep, anoxic seas in recognizing other source rock intervals, and our understanding of the Wufeng and Longmaxi formations may serve as models for future source rock exploration.

On the basis of the results of hydropyrolysis simulations for about 90 different types of immature to mature source rocks selected from about 5 000 marine source rocks in China, along with the natural thermal evolution profiles, the following conclusions were obtained. (1) Total organic carbon (TOC) content of excellent marine source rocks does not change obviously when Ro<0.8% or Ro>1.3%, and the residual TOC content is decreasing gradually with the maturity increase at 0.8%<Ro<1.3%. (2) At the high-post mature stage (Ro>1.3%), the decreased maximums of residual TOC content for the kerogen of sapropel (I), II₁, and II are usually 40%, 32% and 24%, respectively, and their TOC restitution coefficient is, respectively, 1.68, 1.48 and 1.32. (3) Both the TOC decreasing amplitude and the restitution coefficient decrease gradually with the decrease of TOC content for the source rocks with low organic matter abundance (usually 0.3%res. of solid bitumen and shale with high organic matter abundance (TOC>30%) also require no restitution at the high mature stage. Such kind of TOC restitution is further supported by the coincidence of the decrease of residual TOC with the decreasing of S₁+S₂ and the increasing of hydrocarbon quantity during the experimental simulation of hydrocarbon generation and expulsion for marine source rocks (0.3%<TOC<30%) in natural thermal evolution profiles.

In marine strata from Sinian to Middle Triassic in South China, there develop four sets of regional and six sets of local source rocks, and ten sets of reservoir rocks. The occurrence of four main formation periods in association with five main reconstruction periods, results in a secondary origin for the most marine gas pools in South China. To improve the understanding of marine gas pools in South China with severely deformed geological background, the dominant control factors are discussed in this paper. The fluid sources, including the gas cracked from crude oil, the gas dissolved in water, the gas of inorganic origin, hydrocarbons generated during the second phase, and the mixed pool fluid source, were the most significant control factors of the types and the development stage of pools. The period of the pool formation and the reconstruction controlled the pool evolution and the distribution on a regional scale. Owing to the multiple periods of the pool formation and the reconstruction, the distribution of marine gas pools was complex both in space and in time, and the gas in the pools is heterogeneous. Pool elements, such as preservation conditions, traps and migration paths, and reservoir rocks and facies, also served as important control factors to marine gas pools in South China. Especially, the preservation conditions played a key role in maintaining marine oil and gas accumulations on a regional or local scale. According to several dominant control factors of a pool, the pool-controlling model can be constructed. As an example, the pool-controlling model of Sinian gas pool in Weiyuan gas field in Sichuan basin was summed up.

This paper analyzes the quantitative grain fluorescence (QGF) and quantitative grain fluorescence on extract (QGF-E) properties of 101 rock samples by using quantitative grain fluorescence techniques. The samples are collected from five wells in tight sandstone and thin siltstone in the third sector of the Shahejie Formation in the Niuzhuang sag of the Dongying depression. It was observed that both the tight sandstone and thin siltstone show relatively high fluorescence intensity of hydrocarbon, which suggests that they are possibly good subtle oil-migration pathways in the present or geological time. These thin subtle oil-migration pathways afford important clues for the researches on hydrocarbon accumulation in lithological reservoirs in the middle and lower of Es₃ in deep sag zone, which has the hydrocarbon source from the upper of Es₄ when there is no apparent fault playing oil migration conduits to connect lithologic traps and deep source rocks. This study shows good prospect of QGF techniques in discriminating oil migration pathways and paleo-oil zones. The results of this study may be of great significance to the researches on hydrocarbon accumulation mechanism of subtle reservoirs in the Dongying depression and other areas.

Based on studies of regional tectonic setting and the observation of drilling cores, the seismite is recognized in member III at Shahejie Formation in the Xianhe area of the Dongying sag. The principal mark of the seismite is the develop ment of all kinds of soft sedimentary deformation structures, such as micro-step fault, intraformational minor fold, deformation, vibrational liquefied sandstone dike and seismic breccias. By observing the drilling cores of He 152, Wang 59, Niu 38, Niu 22 and so on, we found that the vertical sequences of the seismite are different because of the differences in structure and seismic intensity. We also found that different seismite structures are formed by different seismic intensities and that the relationships between seismite structures and paleoearthquakes were further confirmed by studying the relationship between all kinks of seismite structures and seismic intensity. Researches on reservoirs show that seismite can be an effective reservoir. These researches provide an important geological theoretical foundation for the study of the tectonic evolution of the Dongying sag and for seismite identification and description, and also supplies evidence for the researches of paleoearthquakes.