Oil mobility in liquid-rich shale is crucial for economic success of resource development. The characteristics of the host rock and the properties of oil fluids are two primary factors controlling ...fluid mobility. Organic geochemistry data allows examination of both resource potential and oil mobility. We propose data-driven empirical models in this paper to describe hydrocarbon resource potential with respect to oil mobility. The proposed methods were applied to a dataset collected from the Eocene-Oligocene Es3 and Es4 source rock units in four recent shale oil exploration wells in the Jiyang Super-Depression of the Bohai Bay Basin. The dataset, results from standard programmed pyrolysis of whole rock samples as well as solvent extracted replicates of 139 core samples, was studied to examine the source rock characteristics and shale oil resource potential. Bulk geochemical characteristics and thermal decomposition trends allow for sub-division of the samples into two groups; a) samples with high initial hydrogen index (>800 mg HC/g TOC) and average present day TOC of 4.07% from LY1 and NY1 wells in a distal facies; and b) samples with slightly lower initial hydrogen index (about 700 mg HC/g TOC) and average TOC of 3.11% from FY1 and L69 wells in a transitional facies. Three different categories of oil resource with respect to oil mobility can be defined: adsorbed, restricted and movable based on organic geochemical characteristics and their variations in the stratigraphic intervals represented by the samples in the four wells. We show that expulsion efficiency is a good indicator of hydrocarbon retention in source rock reservoir and that the two wells in the distal facies have higher total resource potentials with abundant movable oil resource than the two wells in the transitional facies. The total oil potential is separated into three types of oil resource to show oil mobility, thus providing more information for business decision.
•Hydrocarbon mobility and retention capacity are crucial in evaluating shale oil resource•Methods are proposed to quantify hydrocarbon retention and mobility in shale resource evaluation•High hydrocarbon retention and better oil mobility associated with distal facies in deep water lacustrine setting
The Lower Jurassic Ziliujing Formation in China’s Sichuan Basin is a significant shale target for exploration; however, the strong heterogeneity of the properties of organic matter (OM) in shale ...makes it challenging to identify the target area for exploration, and the mechanism of OM enrichment is still unclear. Furthermore, the mechanisms of the response of the Da’anzhai member to the Toarcian Oceanic Anoxic Event (T-OAE) are controversial. Previous studies have focused on sedimentary facies analysis based on mineralogy and elemental abundances and have provided minimal information about organic geochemistry, which adds to the challenge of deeply understanding the influence of the T-OAE on the molecular geochemical characteristics of the Da’anzhai member. In this study, the Da’anzhai member of the Lower Jurassic Ziliujing Formation in the Langzhong area, Sichuan Basin, is studied via X-ray diffraction, total organic carbon, gas chromatography–mass spectrometry, organic carbon isotope, organic petrographical and pyrolysis analyses. To accurately identify the trend of the paleosedimentary environmental proxies, the Mann‒Kendall test is utilized to identify the trend of the data. Our results show that the Da’anzhai shale was deposited in a dysoxic transitional environment to an intermittent reducing environment with freshwater to brackish conditions. The response to the T-OAE can be identified in the middle and upper parts of the middle submember and the bottom of the upper submember of the Da’anzhai member. The T-OAE influenced the redox conditions, salinity, and OM origins during deposition in the middle of the Da’anzhai member, which resulted in the enrichment of OM. The abnormally high C30 diahopane/C30 hopane (C30D/C30H) ratio can be considered a potential proxy for locating the section of strata that responded to the T-OAE in the Da’anzhai member. In the study area, the mechanism of the response of the Da’anzhai shale to the T-OAE manifested as an improvement in hydrological cycling rather than a marine incursion. Our study provides new information that deepens the understanding of the mechanisms of the response of lacustrine shales to oceanic anoxic events from the perspective of molecular organic geochemistry.
Obtaining reliable total oil yield in a source rock from routine programed pyrolysis data represents a technical challenge for the characterization of source rocks in early mature and oil window due ...to strong interactions between bitumen and kerogen/rock matrix. A common laboratory solution for a robust estimate of total oil yield requires two pyrolysis experiments using a whole rock sample and a post-solvent extracted replicate. In this paper, we present a numerical solution that provides a reliable estimate of total oil yield directly from a single routine Rock-Eval experiment. In temperature domain, the thermally vaporized products from heavy oil and bitumen adsorbed to the organic matter and the pyrolyzed products from kerogen show a certain degree of overlap in the default temperature range of S2 peak. By transforming the routine Rock-Eval pyrogram (FID curve) into activation energy variable space, it allows for separation of the reactants into subgroups by their responses to ramping temperature. The thermally vaporized products can be then discriminated from the thermally pyrolyzed products based on the differences in their thermal stability and decomposition behaviour. The proposed method was applied to the source rock samples in the Eocene-Oligocene Shahejie Formation of Dongying Depression, Bohai Bay Basin, where programed pyrolysis results from whole rock samples and post-solvent extracted replicates were obtained. The case study suggests that (a) the two-step pyrolysis experiment approach may still under-estimate the total oil yield in case that solvent extraction treatment fails to remove all hydrocarbons in the sample; (b) the post-solvent extracted equivalent S2x values derived numerically from routine Rock-Eval analysis and those derived from post-extracted analysis are comparable with a correlation coefficient of 0.9766, suggesting numerical approach is as effective as additional laboratory experiment; and (c) the proposed approach can remove all petroleum in the sample numerically, thus providing an un-biased and robust estimate of total oil yield, a cost- and time-effective alternative to the traditional two experiment approach.
•Mixing of thermally vaporized bitumens with kerogen cracking products makes estimating total oil yield difficult for early mature rocks•Traditional laboratory approach requires two separate programmed pyrolysis experiments, but still under-estimates the total oil yield•Proposed numerical solution requires only a single routine programmed pyrolysis experiment, providing a cost effective and green alternative
Source rock samples can be “contaminated” by expelled oils from nearby mature source rocks or from the organic-rich laminae within the same source rock units. The “contaminations” on the Rock-Eval ...samples could produce a false high S1 peak, a high Production Index (PI), a suppressed Tmax and other anomalies. Identifying the samples affected by expelled oils, assessing and correcting the impacts are crucial in source rock study for shale oil/shale gas resource evaluation. This study analyzes the Rock-Eval 6 results of 43 core samples from two recent shale oil exploration wells of the Eocene Qianjiang Formation, a confined source rock unit embedded with salt intervals formed in a hypersaline lacustrine setting in central China. The geochemical anomalies of the affected samples are shown through the available samples in this study. Criteria for identifying the affected samples are discussed, and impacts on the conventional Rock-Eval parameters and subsequent estimation of kinetic parameters presented. This study provides insights into Rock-Eval data interpretation, particularly in source rock evaluation for estimating shale oil resources.
•Source rock samples can be contaminated by oils expelled from adjacent organic-rich rocks•Exogenous hydrocarbons in confined source rocks may affect Rock-Eval data interpretation•Identifying the affected samples will help improve Rock-Eval data interpretation
•Experiments on the oil adsorption on minerals were carried out.•A model was developed to reconstruct the adsorption of mudstone/shale for shale oil.•The shale oil exploration risks rise at depth ...shallower than 3400m.
Inorganic minerals were separated from mudstone and shale to investigate their oil adsorption potential. A conceptual model was developed to reconstruct the oil adsorption capacity of underground mudrock/shale by combining adsorption and organic matter swelling data. The predictions for the free hydrocarbon oversaturation sorption zones in the Well NY1 profile were taken as a case study. The results suggest that the best depth for the shale oil prospect of the Es4s member strata lies between 3400 and 3600m in the Dongying Depression. Hydrocarbon starvation zones occur mainly at a depth of <3400m, where both the maturity and the extent of oil conversion of organic matter are relatively low, while the sorption capacity of mudstone and shale rocks is by comparison high and the exploration risk is greater.
•Neutral nitrogen compounds occur mainly in expelled oil/free bitumen.•O-containing functional groups facilitate bitumen-rock matrix interactions.•Organic acids released during maturation enhance ...secondary porosity formation.
To evaluate petroleum generation and expulsion processes, pyrolysis experiments that simulate increased overburden pressures were conducted on an immature organic-rich shale with type-II kerogen. Expelled oils and residual rocks with different maturities covering the oil window were recovered from the experiments. The residual rocks were then subjected to stepwise extraction in conjunction with stepwise dissolution of the carbonate and silicate minerals to obtain free and bound bitumen fractions. The compositions of acidic and neutral compounds in both expelled oils and bitumen fractions were investigated by negative-ion electrospray (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Neutral nitrogen compounds were found to only exist in expelled oil and free bitumen, while acidic compounds (typically contain carboxylic functional group(s)) were prevalent in bound bitumen. Oxygen-containing functional groups with high polarity in oxygen-containing compounds and oxygenated nitrogen/sulfur compounds are proposed to be the dominant bonding forces that facilitate the interaction between bitumen and mineral matrix or kerogen. Semiquantitative results of acidic compounds reveal that the distributions of acidic compounds in both free and bound bitumens vary throughout the oil window stage, especially for those in the bound phase after an intense oil expulsion from the shale at Easy%Ro > 0.9. During maturation, acidic compounds in free and bound bitumens gradually decreased in their abundances. However, oil expulsion from the shale brought about a significant redistribution of acidic compounds among expelled oil, free and bound bitumens. These results indicate that polar components in shales are closely related to thermal maturation of organic matter and oil expulsion from the shales in a petroleum system.
•Kerogen and expelled oil show opposite compositional changes during shale maturation.•Thermal alteration and expulsion fractionation control shale oil content and quality.•Knowledge gained from this ...study improves the understanding of retention mechanisms.
Knowledge of the formation and enrichment mechanisms of shale oil is essential to understanding its mobility and hence a critical aspect for evaluating shale oil exploration and production. Semi-open hydrous pyrolysis experiments were performed on an organic-rich shale to simulate the petroleum generation and expulsion processes of Type II kerogen. The residual rocks matured to different levels (Easy%Ro = 0.50–1.37) were thereafter treated with stepwise extraction to obtain free and bound bitumen fractions. This study focuses on the structural characteristics of kerogen, bitumen fractions, and expelled oil during shale maturation using Fourier transform infrared (FTIR) spectroscopy and quantitative flash pyrolysis–gas chromatography (Py–GC). The results reveal that the composition of expelled oils changes with increasing maturity, whereas an opposite trend in structural composition was observed for kerogen after significant oil expulsion from the shale occurs, beginning at Easy%Ro = 0.79. A notable change is that aliphatic carbon chains of kerogen became shorter and/or more branched while those of expelled oil appeared to be more aliphatic with longer carbon chains at this stage. However, the overall compositional structures of shale bitumen fractions, both in free and bound phases, show only slight changes within the oil window stage. The bound bitumen is generally enriched in oxygenated functional groups as compared to free bitumen, indicating that the bound bitumen is related mainly to the interaction between the mineral surface and the bitumen–kerogen matrix. During maturation, the cracking (including defunctionalization) and condensation of kerogen and bitumen fractions, as well as a compositional fractionation due to oil expulsion, are likely to be the main factors influencing the opposite trend observed in structural compositions between kerogen and expelled oil. These processes may control the quality and content of expelled oil and bitumen in a petroleum system. Nevertheless, kerogen, expelled oil and bitumen show a gradual trend of decarboxylation during shale maturation in general.
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.
Display omitted
•A unified model for the formation and distribution of both conventional and unconventional reservoirs.•Conventional reservoirs in free hydrocarbon dynamic field with high porosity and permeability.•Unconventional tight reservoirs in confined hydrocarbon dynamic field with low porosity and permeability.•Shale oil/gas reservoirs in the bound hydrocarbon dynamic field with low porosity and ultra-low permeability.
•Quantitative oil shale maceral concentrates were obtained from an Eocene Huadian oil shale.•Density separated maceral groups had different petroleum generation capacities.•The properties of end ...member lamalginite and detrovitrinites properties were determined for maceral isolates.•Quantitative retorting behavior can be predicted using precise maceral distributions.•Lamalginite generates more hydrocarbons but with a higher activation energy.
Oil shales are an important alternative energy resource, through retorting, for the production of liquid hydrocarbons. A key factor in determining the value of an oil shale resource is its hydrocarbon generation potential; particularly its maceral distribution. To examine this, single maceral concentrates were obtained from a 1 kg Eocene Huadian oil shale sample using float-sink separation of chemically demineralized kerogen. Nine different density fractions were separated: the Botryococcus fraction occurred in the lightest (<1.06 g/mL) material, lamalginite in the 1.06–1.23 g/mL fraction, while detrovitrinite dominated in a higher density fraction (1.26–1.36 g/mL), which also contained some lamalginite.
Rock-Eval S2 and hydrogen index (HI) parameters, as well as the aliphatic compound peaks in Fourier transform infrared (FTIR) spectra, all decreased with increasing kerogen density, whereas the Rock-Eval oxygen index and the aromatic compound peaks in FTIR spectra, increased. The δ13C values showed slight variations within the different density fractions. The δ13C value of parent shale kerogen was very close to that of the density fraction dominated by lamalginite (−25.1‰), because the sample is dominated by lamalginite macerals (>70%). The Botryococcus sp. fraction is isotopically enriched because Botryococcus sp. is typically significantly enriched in 13C (δ13C = −24.6‰).
Artificial thermal maturation hydrocarbon generation results show that absolute hydrocarbon gas yields were similar among the different density fractions. The 1.06–1.12 g/mL fraction, dominated by lamalginite, generated nearly twice as much liquid hydrocarbons as the 1.26–1.30 g/mL fraction dominated by detrovitrinite. The maximum hydrocarbon yields corresponded well with the Rock-Eval HI values. Calculated by the relationships among HI, detrovitrinite and alginite compositions, the HI is 909 and 177 mg HC/g TOC for the purified alginite and detrovitrinite fractions, respectively. Kinetics analysis shows that Ea (activation energy) is higher for the low-density fraction (1.06–1.12 g/mL) than that for the heavy fraction (1.26–1.30 g/mL), indicating that lamalginite not only has a much better hydrocarbon generation potential, but also higher thermal stability. Therefore, this lamalginite-rich oil shale is well suited for exploitation by retorting, even though the retorting temperature would be higher than for detrovitrinite-rich fractions.
This article presents a novel numerical decomposition method based on generation kinetics for studying oil composition characteristics from S1 peak of single heating rate pyrolysis of source rock ...samples. Furthermore, phase behavior and PVT analysis were introduced in the calculation to make a full correction for evaporative loss of gaseous and light hydrocarbon in S1 during coring and sample storage. Special programed pyrolysis results of twenty-eight core samples, along with a routine source rock screening pyrolysis dataset of 68 samples, from a recent shale oil study well of the Eocene Qianjiang Formation in Jianghan Basin, China were analyzed to demonstrate the application of proposed methods. We showed that (a) by reassembling corresponding activation energies, the original S1 curve can be split into groups of similar thermal evaporative property, resulting in a composition classification of free hydrocarbons in S1 when it was converted back to temperature domain; (b) a full correction on the gaseous and light hydrocarbon loss in S1 permits a more precise estimation of oil composition as well as total resource; (c) the resulting oil composition grouping provides information for inferring the fraction of extractable hydrocarbon; d) oil composition varies considerably among the samples within a stratigraphic interval of 20 m. Samples from intra salt organic-rich lamina show different compositional characteristics with much less abundance in gaseous and volatile oil compounds; while samples from inter salt interval contain large portion of volatile and light oil, indicating a promising production zone.
•Thermal evaporative kinetics of S1 curve was used to characterize oil composition groups•Oil compositional grouping provides essential information for assessing oil mobility and extractable oil resource•Phase analysis based correction of evaporative loss in S1 led to insights for realistic oil composition characterization
PDF