Low-mature Gases Research Articles

Genetic types and origins of natural gases from eastern Fukang Sub-depression of the Junggar Basin, NW China: Implication for low-mature coal-derived gases

Although the Fukang Sub-depression is the largest hydrocarbon generation depression in the Junggar Basin and promises favorable potential for oil and gas exploration, only several small–medium oil/gas accumulations have been discovered so far. After analyzing the molecular and stable carbon isotopic compositions of natural gases and its associated low molecular weight hydrocarbons (LMWHs) from eastern Fukang Sub-depression, their genetic types and origins are discussed in this study. Natural gases are wet gases with an average dryness coefficient of 0.84. Stable carbon isotopes of methane and ethane are well correlated with the thermal evolution trend of coal-derived gases. The C5–7 LMWHs are enriched in methyl cyclohexane but they lack n-alkanes, indicating a predominance of higher plant input. Gases have maturities ranging 0.76%–0.93%Ro and are well consistent with their low n-heptane (H) and iso-heptane (I) contents. The geochemical characteristics of natural gases from eastern Fukang Sub-depression are quite similar with those from the Turpan–Hami Basin. Their gases are considered to be low-mature gases derived from the Lower–Middle Jurassic coal measures. In contrast, they show remarkable differences with those from the Wucaiwan gas field; they are considered to have high-mature gases derived from the Carboniferous coal measures.

Modeling of microbial gas generation: application to the eastern Mediterranean “Biogenic Play”

Biogenic gas is becoming increasingly important as an exploration target in the petroleum industry because it occurs in geologically predictable circumstances and in large quantities at shallow depths as free gas or gas hydrates. As accumulations of biogenic gas result in a subtle synchronization between early generation and early trapping, we integrated a macroscopic model of microbial gas generation within a 3D basin and petroleum system forward simulator. The macroscopic model is based on a microscopic model, which consists in a 1D sedimentary column that accounts for sedimentation, compaction, Darcy flow and Diffusion flow. The organic carbon is the only non-soluble element considered in this version of the model. The dissolved elements are O2, SO4 2-, H2, CH3COOH, and CH4. Methane is dissolved in water or present as a free phase if its concentration exceeds its solubility at given pressure and temperature. In this microscopic model, the transformation of substrate into biomass is described through a set of logistic equations coupled with the transport equations (advection and diffusion). Based on the microscopic considerations we developed the macroscopic model of low maturity/biogenic gas generation in which hydrocarbons are generated through first order kinetic reactions at low maturity. This macroscopic model is adapted to petroleum system modeling at basin scale with TemisFlow®, which aims to understand and predict hydrocarbon generation, migration, and accumulation. It is composed of: i) A source rock criteria which allow defining the biogenic gas source rocks potential and ii) A kinetic model of methane generation. The previous model has been successfully applied on different basins such as the Carupano Basin from the offshore Venezuela, the Magdalena Delta (offshore Colombia) and the offshore Vietnam where direct observations of low-maturity gas were available. Furthermore, it has been applied in the offshore Lebanon in order to check the viability of a biogenic gas system

Origin and accumulation mechanism of shallow gases in the North Baodao Slope, Qiongdongnan Basin, South China Sea

Abstract On the basis of geochemical characteristics and origins of gases in the North Baodao slope of the Qiongdongnan Basin, the gas migration and accumulation episodes and reservoir forming time are investigated and the reservoir forming model is built for the BD13 shallow gas pools. The chemical composition and carbon isotopic data of the shallow gas, are compared with the characteristics of BD19-2 high-mature gas from the south section of the same structural belt, which reveals that the shallow gas of the BD13 gas pools is a mixture of biogas/low mature gas and some mature gas. In combination with the associated condensate biomarkers and the inclusions data, it is inferred that the biogas and low mature gas in the BD13 gas pools came from the nearby immature to low mature source rocks of the Miocene to Oligocene and charged approximately 3 Ma ago; whereas the high-mature gas generated by the Yacheng Formation source rock in the Badao Sag migrated lately into the reservoirs. Geochemical and geological data show that the No.2 fault system acts as the key pathway for upward gas migration into the shallow reservoirs, and that the sand-bodies within the Miocene Meishan and Sanya Formations are the main conduits for lateral migration of the gas. This gas migration pattern implies that the Songdong-North Baodao slope has favorable gas exploration potential in the future gas exploration of the east shallow water area in the Qiongdongnan Basin.

Characteristic of homodrimane and maturity of organic matter in the low mature source rocks

Bicyclic sesquiterpenoids are mostly used to illustrate the origins of organic matter and genetic types of crude oil presently. In this paper, the relationship between distributions and compositions of C16 homodrimane in immature to mature source rocks and evolution of organic matter was studied, and the correlation between C16 homodrimane at immature to low mature stages and thermal evolution of organic matter was analyzed. The results show that in terrestrial facies depositional system, the ratio of 8α(H)-homodrimane to 8β(H)-homodrimane has a high sensitivity about the maturity’s minor changes at immature to low mature stages. It is found that when the vitrinite reflectance R0<0.7%, the ratio significantly decreases with increasing burial depth or maturity. This kind of phenomenon reveals that these parameters may be the useful maturity indicators for determining the relative maturation of organic matter at immature to low mature stages, and have certain practical value in biogeochemical and environmental geochemical research on low mature oil and gas.

Geochemical characteristics of light hydrocarbons in natural gases from the Turpan-Hami Basin and identification of low-mature gas

Theoretical and practical knowledge regarding low-mature gasses is of significant importance to identifying potential natural gas resources. Light hydrocarbon parameters and C and H isotopes are useful tools to identify low-mature gas. Twenty gas samples were collected from the Turpan-Hami Basin for light hydrocarbon analyses. The results showed that the light hydrocarbon components of natural gases contain high methylcycloxane, high isoparaffin and low benzene. This implies that the gas-generating parent materials are of typical humus type and the paleoenvironment is a fresh water sedimentary environment. These features are consistent with the geological setting of the basin. Comparative studies of isoheptane, heptane, and the carbon isotopic compositions of methane in natural gases, and other maturity indices indicated that natural gases in the Turpan-Hami Basin are dominated by low-mature gas formed during the low evolution stage of Jurassic coal seams. The parent materials are of type III, and the maturation degree was in the low evolution stage. These are the fundamental characteristics of low-mature gas. Results of light hydrocarbon research provided further evidence to suggest that the Turpan-Hami Basin is a large-scale gas producer of low-mature gas in China. It is likely that this resource will play an important role in future exploration and development of low-mature gas in China.

Genetic types and distribution of shallow-buried natural gases

Great volumes of shallow-buried (<2,000 m) natural gases which are mainly composed of biogases and low-mature gases have been found in the Mesozoic-Cenozoic sedimentary basins in China. Many shallow gas reservoirs in China are characterized by coexistence of biogas and low-mature gas, so identifying the genetic types of shallow gases is important for exploration and development in sedimentary basins. In this paper, we study the gas geochemistry characteristics and distribution in different basins, and classify the shallow gas into two genetic types, biogas and low-mature gas. The biogases are subdivided further into two subtypes by their sources, the source rock-derived biogas and hydrocarbon-derived biogas. Based on the burial history of the source rocks, the source rock-derived biogases are divided into primary and secondary biogas. The former is generated from the source rocks in the primary burial stage, and the latter is from uplifted source rocks or those in a secondary burial stage. In addition, the identifying parameters of each type of shallow gas are given. Based on the analysis above, the distributions of each type of shallow gas are studied. The primary biogases generated from source rocks are mostly distributed in Quaternary basins or modern deltas. Most of them migrate in watersoluble or diffused mode, and their migration distance is short. Reservoir and caprock assemblages play an important role in primary biogas accumulation. The secondary biogases are distributed in a basin with secondary burial history. The oil-degraded biogases are distributed near heavy oil pools. The low-mature gases are widely distributed in shallow-buried reservoirs in the Meso-Cenozoic basins.

Geochemical characteristics and origin of light hydrocarbons in biogenic gas

The light hydrocarbon geochemical characteristics of biogenic gases from Sebei 1 gas field in the Qaidam Basin, Baoshan gas field in the Baoshan Basin and Alaxin gas field, Puqian gas pool, Aonan gas pool in the Songliao Basin are studied and the origin is discussed based on the composition and isotope data of gases. The isoalkane contents among light hydrocarbons in natural gas show a negative relationship with δ13C1 values. The isoalkane contents of the gases with δ13C1 values of less than −60‰ are also high with more than 40% among light hydrocarbons in Sebei 1 gas field and Puqian gas pool. Moreover, the 2,2-dimethylbutane and 2-methylpentane, mainly sourced from bacteria, have predominance among isoalkanes, which suggests that light hydrocarbons in biogenic gases from these gas fields or pools were probably generated by microbial action. However, the cycloalkane contents among light hydrocarbons in biogenic gas are related to δ13C1 values positively. In Alaxin gas field and Aonan gas pool, where δ13C1 values of biogenic gases are less than −60‰, the average contents of cycloalkane are higher than 44%. Light hydrocarbons among biogenic gases from these gas fields were probably generated by catalysis. The isoalkane and cycloalkane contents among light hydrocarbons from biogenic gases in the Baoshan gas field are both high, which might be generated by these two actions. The results show that the data of light hydrocarbons in biogenic gas can provide important information for understanding the generation mechanisms of light hydrocarbons during geological evolution and identifying biogenic gas and low mature gas.

Low-mature gases and their resource potentiality

In the 80’s of last century, based on the advances in natural gas exploration practice, the concepts of bio-thermocatalytic transitional-zone gas and early thermogenetic gas were proposed, and the lower limit Ro values for the formation and accumulation of thermogenetic natural gases of industrial importance have been extended to 0.3%–0.4%. In accordance with the two-stage model established on the basis of carbon isotope fractionation involved in the formation of coal-type natural gases, the upper limit Ro values of lowly evolved natural gases should be set at 0.8%–1.0%. This is the concept of low-mature gas which is commonly accepted at the present time. The Urengoy super-large gas field in western Siberian Basin is a typical example of low-mature gas field, where low-mature gas reserves account for 20% of the globally proven natural gas reserves, and this fully indicates the importance of this kind of resources. The proven reserves of natural gases in the Turpan-Hami Basin of China are approximate to 1000×108 m3, and the thermal evolution indices of source rocks are Ro=0.4%–0.8%. The δ13C1 values of methane are mainly within the range of −44‰–39‰ (corresponding to Ro=0.6%–0.8%), and those of ethane are mainly within the range of −29‰–26‰, indicating that these natural gases should be designated to the coal-type low-mature gases. The light hydrocarbon evolution indices of natural gases also provide strong evidence suggesting that they are the coal-type low-mature gases. If so, low-mature gas in the Turpan-Hami Basin has been accumulated to such an extent as to be equivalent to the total reserves of three large-sized gas fields, and their existence is of great significance in the study and exploration of China’s low-mature gases. If it is evidenced that the source rocks of low-mature gases are related mainly to coal measures, China’s abundant lowly evolved coal series resources will provide a huge resource potentiality for the generation of low-mature gases.

Low-mature gases and typical low-mature gas fields in China

No natural gas pool of industrial importance could be formed at the low-evolution stage of organic matter. In the 1980s, on the basis of the development in exploration practice, the hypotheses of bio-thermo-catalytic transitional zone gases and early thermogenic gases were proposed. The lower-limit Ro values for the formation and accumulation of natural gases of industrial importance have been expanded to 0.3%–0.4%. In the light of the two-stage model established on the basis of carbon isotope fractionation in coal-type natural gases, the upper-limit Ro values have been set at 0.8%–1.0%. In terms of the geological practice in the low-mature gas zones and China’s main coal-type gas fields, it is feasible and proper to set the upper-limit Ro value of low-mature gases at 0.8%. Supper-large gas fields such as the Urengoy gas field in western Siberian Basin should belong to low-mature gas fields, of which the natural gas reserves account for more than 20% of the global proven reserves, providing strong evidence for the significance of such a type of resources. The proven natural gas reserves in the Turpan-Hami Basin of China have almost reached 1000 ×108 m3. The main source rocks in this area are the Jurassic Xishanyao Formation, which occurs as a suite of coal series strata. The corresponding thermal evolution indices (Ro) are mainly within the range of about 0.4%–0.8%, the δ13C1 values of methane vary between −44‰ and −39‰ (correspondingly Ro =0.6%–0.8%), and those of ethane are within the range of −29‰–−26‰, indicating that natural gases in the Turpan-Hami Basin should be designated to coal-type low-mature gases. The light hydrocarbon evolution indices of natural gases also fall within the area of low evolution while the precursor type of light hydrocarbons also shows the characteristics of the coal-type. The geological background, carbon isotopic composition and light hydrocarbon index all provide strong evidence suggesting that the proven natural gases in the Turpan-Hami Basin are low-mature gases. In China a gas field with the gas reserves reaching 300 ×108 m3 can be defined as a large gas field, and thus the proven low-mature gases in the Turpan-Hami Basin are equivalent to the reserves of three large gas fields. Its existence is of great significance in research on and exploration of low-mature gases in China.

Oxidant-elemental sulfur as an effective promoter for transformation of organic matter to hydrocarbons at moderate-low temperatures

Elemental sulfur is widely dispersed in the hydrocarbon source rocks and its depositional environment is usually thought as a reducing environment. The presence or absence of free oxygen is a key to identify oxidizing or reducing environment. But elemental sulfur is often present as an oxidant in this environment. When elemental sulfur meets with organic matter, redox reaction will occur. In our simulation experiments at 200 – 400°C, the existence of elemental sulfur can sharply increase the amounts of hydrocarbons, hence leading to the production of immature or low-mature oils and natural gases. At the temperature of 300°C, the addition of elemental sulfur will further enhance the relative yields of hydrocarbons, and the final yield of total extracts and gaseous hydrocarbons of similitude kerogens by more than 463% and 2760%, respectively, while those of oil shales are increased by about 71% and 2044%, respectively. But at the temperature of 450°C, elemental sulfur plays a negative role in liquid hydrocarbon formation. The presence of elemental sulfur is probably a key factor in the gypsolyte environment leading to the formation of immature or low-mature oils, as well as the coexistence of immature or low-mature oils and natural gases.

Genetic identification of natural gases from shallow reservoirs in some oil- and gas-bearing basins of China

Natural gases of shallow reservoirs with the carbon isotopic compositions of methane ranging from −50‰ to −60‰ (PDB) were considered as mixed gases of biogenic and thermogenic origins previously and some of them were considered as low-mature (or low temperature thermogenic) gases lately. In this paper natural gases with the carbon isotopic compositions of methane in the above range were identified using the molecular and stable carbon isotopic compositions of methane, ethane and propane. The mixed gases of biogenic and mature thermogenic origins display the characteristics of δ13C1 ranging from −50‰ to −60‰, δ13C2>−35‰, Δ values (δ13C3 - δ13C2)<5‰ and C1/ΣC 2 + ratios 7‰, and C1/ΣC 2 + ratios >60.

Laboratory and theoretical constraints on the generation and composition of natural gas

Hydrous pyrolysis experiments were conducted at 125 to 375°C and 350 bars to constrain factors that regulate the generation and relative abundance of hydrocarbon and nonhydrocarbon gases during thermal maturation of Monterey, Eutaw, and Smackover shale. Thermogenic gas was generated at temperatures as low as 125°C and increased in abundance with increasing temperature. The relative abundance of individual hydrocarbons varied substantially in response to increasing time and temperature reflecting the chemical processes responsible for their formation. The hydrocarbon fraction of low maturity gas produced via primary cracking of kerogen was composed predominantly of methane. With increasing thermal maturity, the onset of bitumen generation produced longer-chain hydrocarbons causing a decrease in the relative abundance of methane. At high levels of thermal maturity, the absolute and relative abundance of methane increased due to decomposition of bitumen. In all experiments at all temperatures, carbon dioxide was the most abundant volatile organic alteration product. Carbon dioxide was produced directly from kerogen at low thermal maturity and via the decomposition of bitumen and/or kerogen at high thermal maturity. During early stage alteration, kerogen likely represents the dominant source of oxygen in carbon dioxide while at high thermal maturities water may represent an abundant and reactive oxygen source. Hydrogen released during the disproportionation of water is likely consumed during hydrocarbon generation. Theoretical reaction path modeling suggests that the precipitation of calcite may effectively remove carbon dioxide from natural gas if a source of Ca is available within the rock. Thus, carbon dioxide-rich natural gas may be relatively pristine while methane-rich natural gas may reflect the occurrence of secondary reactions involving inorganic sedimentary components. Kinetic analysis of the experimental data indicates a narrow range of activation energies for the generation of C 1-C 4 hydrocarbons from the Monterey, Smackover, and Eutaw shales. Carbon dioxide generation from the Monterey and Eutaw shales is accounted for by a substantially broader range of activation energies. Application of these data to predict gas formation at temperatures and time scales typical of subsiding sedimentary basins suggests that C 1-C 4 generation is restricted to relatively high temperatures while carbon dioxide generation occurs at both low and high thermal maturities. Thus, in contrast to the bulk of C 1-C 4 generation which is predicted to occur after peak bitumen generation, production of carbon dioxide will occur before, during, and after the generation of liquid hydrocarbons.