Zhaoyun Wang

Further discussion on the connotation and significance of the natural gas relaying generation model from organic matter

Abstract The natural gas relaying generation model (abbreviated as relay model) of organic matter focuses on the potential of gas generation and accumulation derived from the dispersed liquid hydrocarbons retained in source rocks during high to over mature stage (Ro>1.6%). It means the gas generation converts from thermal degradation of kerogen to oil cracking with the increase of thermal maturation, and it forms relaying gas generation processes at different thermal maturation. According to the supplementary kinetic studies and anatomization of discovered gas reservoirs, it was found that source rocks still contain a large amount of liquid hydrocarbons within oil window even after expulsion, which can serve as the effective gas source kitchen during the high to over mature stage. The maturity for the gas largely generated from oil cracking is greater than 1.6% vitrinite reflectance (Ro), and the content of methyl cyclohexane can be utilized as a specific index for identification of such natural gas. Conventional outside source natural gas reservoirs with considerable scale could be formed due to the existence of migration pathway, driving force and process for effective expulsion under high diagenetic environment. The significance of the natural gas relay model is reflected at the following three aspects: (1) focusing on natural gas accumulations of dispersed liquid hydrocarbons retained in source rocks, (2) determination of the main gas generation periods of retained liquid hydrocarbons, and (3) evaluation on the conventional gas accumulation potential of the cracking gas outside of the source kitchen from the detained liquid hydrocarbons during the high to over mature stage.

Geochemical characteristics of natural gas from mud volcanoes in the southern Junggar Basin

A number of mud volcanoes exist in the southern Junggar Basin. To date few systematic studies on natural gas geochemistry of mud volcanoes have been conducted in China. In June 1991 and August 2010, the authors investigated the mud volcanoes in the southern Junggar Basin twice, and the mud volcanism weakened gradually as seen from the variations such as the decrease of gas pressure and output, the downthrow and dry up of the mud pool. The volcanic intensity was significantly weaker than that in Taiwan. The natural gas from the mud volcanoes in the southern Junggar Basin has similar geochemical characteristics, indicating the same source or origin. The main component of the mud volcano gas is alkane gas with contents of 91.15%–97.49%, and the gas is high-quality commercial gas since methane dominates in the alkane. The δ13C1 values are −49.1‰–−40.6‰, which are in accordance with the peak δ13C1 frequency of mud volcano gas around the world, and the alkane gas displays positive carbon isotopic series, i.e., δ13C1<δ13C2<δ13C3, suggesting typical thermogenic origin. The helium in the mud volcano gas is typically crust-derived due to the low R/Ra values of 0.011–0.054. The mud volcano gas is coal-derived since the δ13C2 values are all greater than −28‰, and C1/C1–4 and δ13C1 values are in accordance with those of natural gas derived from the Lower-Middle Jurassic coal-measures. Therefore, alkane gas from mud volcanoes in the southern Junggar Basin is mainly sourced from the Lower-Middle Jurassic coal-measures.

Principal characteristics and forming conditions for medium-low abundance large scale oil/gas fields in China

Abstract In recent years, some of large scale oil or gas fields with medium-low abundance have been discovered in onshore China. They have such characteristics in common as low porosity and permeability, small oil or gas column, low reserves abundance, and large scale. Studies conducted by the authors show that the formations of large-area reservoirs with medium-low abundance have the following favorable conditions: the large scale sandbodies caused by drag flow in the hinterland of large lacustrine basins, alternating broadly with source rocks like “sandwich-type” architecture of reservoir-source rock association, provide fundamental basis of oil/gas accumulation in large scale; the small oil or gas column and normal to low pressure system reduce the restrict requirement on cap-rock quality, which leads to large scale reservoir formation; the strong reservoir heterogeneity with poor connectivity minimizes gas escape energy and ensures wide oil/gas accumulations even in the area with relatively poor geological conditions; the late uplifting and unloading environment makes it possible for source rock desorb and discharge hydrocarbon to form reservoirs with young generation. Studies indicate that there are nice prospects in discovering large scale lithological reservoirs in the synclinal areas of large terrestrial depression basins of China. This concept has guided the oil/gas discoveries in synclinal areas of Ordos, Songliao, and Sichuan Basins, promoted the great increase of oil and gas reserves and shown a good potential for future exploration.

Oil cracking: An important way for highly efficient generation of gas from marine source rock kitchen

The potentials of gas generation by kerogen in the late period and by crude oil cracking are closely related to the origin of natural gas in the high-to over mature marine area and their exploration perspectives. The carbon structure of kerogens, with different types and at different evolution stages, have been experimentally studied using the high magnetic field solid13C nuclear magnetic resonance technique in order to determine the oil and gas potential of kerogens. Results show that the contents of gas potential carbon(GPC) of types I, II, III kerogens at the high-to over mature stage are very low, indicating their weak gas-generating capacity and limited gas production; however, the content of oil potential carbon(OPC) of the low mature type I kerogen is much higher, implying that a large amount of crude oil generated during the oil-generating period will be the material for later gas generation by oil cracking. The kinetic experiment of gas generation by crude oil cracking shows that, when the temperature is about 160°C (Ro=l.6%), the crude oil will start to produce large amounts of gas; the temperature range for major gas generation of crude oil is higher than that of the kerogens, and the gas production is 2 to 4 times higher than that of kerogens. The natural gas derived from oil cracking (called oil-cracked gas) is much abundant in methyl hexamethylene, which is quite different from the natural gas produced by thermal degradation of kerogens (named kerogen degradation gas) at high-to over mature stage.

Large-scale accumulation and distribution of medium-low abundance hydrocarbon resources in China

Abstract This paper analyzes the large-scale accumulation conditions and distribution characteristics of medium-low abundance hydrocarbon resources in China. Large-scale development of accumulation elements and their change in scale are the material basis of large scale oil and gas accumulation, determining the regional nature of oil and gas distribution. Liquid hydrocarbon dispersed in marine source rocks being cracked to form a large volume of gas and coal measure source rocks expelling gas during uplift are two important factors for the formation of large-scale hydrocarbon accumulation, which control the scale of source rocks that enter the main gas-generating stage. Volume flow and diffusive flow are the main migration-accumulation mechanism for the large-scale hydrocarbon accumulation, which ensures the sufficiency of hydrocarbon supply. Pancake, layer-like, and cluster are three main accumulation forms of large-scale hydrocarbon accumulation, which ensure the scale of hydrocarbon accumulation. Middle to low abundance hydrocarbon resources are characterized by near-source distribution, main-body play, late accumulation stage and single accumulation type. The periclinal area of palaeo-highs in marine craton basins, the lower slopes and sags in an intra-continental depression basin, and the gentle slopes of foreland basins are the most likely areas for the development of large-scale hydrocarbon accumulation, and they have two types of accumulation, large area and large scope. The proposal of the large-scale accumulation of middle to low abundance hydrocarbon resources in China improves the hydrocarbon discovering potential in middle to deep layers of superimposed basins and in the lower slopes and sags in depression basins, enlarges the exploration scale, and extends the hydrocarbon exploration from local second-order structure zones to the whole basin with the main source rock as the center, and from middle layers to deep, even super-deep, layers.

Contribution and significance of dispersed liquid hydrocarbons to reservoir formation

Abstract Oil expulsion rate of different types of source rock in geologic settings are estimated by modeling hydrocarbon generation and expulsion and examining the geological sections of different source-reservoir combinations. The major gas generation peak of oil-cracked gas kitchen is determined by study on the gas generation timing of dispersed liquid hydrocarbon inside and outside source rock and ancient oil reservoirs, and the factors affecting the gas generation timing. A “five-step method” based on the origin method is set up to quantitatively evaluate the cracked gas generated by dispersed liquid hydrocarbons. The hydrocarbon generation and expulsion experiment of different types of source rock, the study on oil expulsion rate of different source-reservoir combinations, analysis of rock pyrolysis parameters of reservoir rock all show that there is a large amount of liquid hydrocarbons retained in the source rock, and the feature on the whole is the oil expulsion rate decreases with the reducing of total organic carbon (TOC). The expulsion rate of source rock with TOC of less than 2% is below 50% in general during the liquid-window stage, while that of alternating sandstone and mudstone and of thick mudstone, with TOC of 2%−4%, are 60% and 30% respectively. Affected by the ancient landform, the expelled liquid hydrocarbons accumulated in different abundance in formations, with dispersed or semi-dispersed and semi-gathered form, and they are called dispersed liquid hydrocarbons outside source rock. When buried deeper, both dispersed liquid hydrocarbons inside and outside the source can be cracked into natural gas to form conventional and unconventional gas accumulations. Exploration practices in the Sichuan and Tarim basins have proven that the cracked gas generated by dispersed, semi-dispersed and semi-accumulated, and accumulated liquid hydrocarbons is an important source of deep natural gas in China, and takes an important position in gas exploration.