Tongwei Zhang

Organic matter–hosted pore system, Marcellus Formation (Devonian), Pennsylvania

The Marcellus Formation of Pennsylvania represents an outstanding example of an organic matter (OM)–hosted pore system; most pores detectable by field-emission scanning electron microscopy (FE-SEM) are associated with OM instead of mineral matrix. In the two wells studied here, total organic carbon (TOC) content is a stronger control on OM-hosted porosity than is thermal maturity. The two study wells span a maturity from late wet gas (vitrinite reflectance [Ro], 1.0%) to dry gas (Ro, 2.1%). Samples with a TOC less than 5.5 wt. % display a positive correlation between TOC and porosity, but samples with a TOC greater than 5.5 wt. % display little or no increase in porosity with a further increasing TOC. In a subset of samples (14) across a range of TOC (2.3–13.6 wt. %), the pore volume detectable by FE-SEM is a small fraction of total porosity, ranging from 2 to 32% of the helium porosity. Importantly, the FE-SEM–visible porosity in OM decreases significantly with increasing TOC, diminishing from 30% of OM volume to less than 1% of OM volume across the range of TOC. The morphology and size of OM-hosted pores also vary systematically with TOC. The interpretation of this anticorrelation between OM content and SEM-visible pores remains uncertain. Samples with the lowest OM porosity (higher TOC) may represent gas expulsion (pore collapse) that was more complete as a consequence of greater OM connectivity and framework compaction, whereas samples with higher OM porosity (lower TOC) correspond to rigid mineral frameworks that inhibited compactional expulsion of methane-filled bubbles. Alternatively, higher TOC samples may contain OM (low initial hydrogen index, relatively unreactive) that is less prone to development of FE-SEM–detectable pores. In this interpretation, OM type, controlled by sequence-stratigraphic position, is a factor in determining pore-size distribution.

Experimental investigation on the carbon isotope fractionation of methane during gas migration by diffusion through sedimentary rocks at elevated temperature and pressure

Molecular transport (diffusion) of methane in water-saturated sedimentary rocks results in carbon isotope fractionation. In order to quantify the diffusive isotope fractionation effect and its dependence on total organic carbon (TOC) content, experimental measurements have been performed on three natural shale samples with TOC values ranging from 0.3 to 5.74%. The experiments were conducted at 90°C and fluid pressures of 9 MPa (90 bar). Based on the instantaneous and cumulative composition of the diffused methane, effective diffusion coefficients of the 12CH4 and 13CH4 species, respectively, have been calculated. Compared with the carbon isotopic composition of the source methane (δ13C1 = −39.1‰), a significant depletion of the heavier carbon isotope (13C) in the diffused methane was observed for all three shales. The degree of depletion is highest during the initial non-steady state of the diffusion process. It then gradually decreases and reaches a constant difference (Δ δ = δ13Cdiff −δ13Csource) when approaching the steady-state. The degree of the isotopic fractionation of methane due to molecular diffusion increases with the TOC content of the shales. The carbon isotope fractionation of methane during molecular migration results practically exclusively from differences in molecular mobility (effective diffusion coefficients) of the 12CH4 and 13CH4 entities. No measurable solubility fractionation was observed. The experimental isotope-specific diffusion data were used in two hypothetical scenarios to illustrate the extent of isotopic fractionation to be expected as a result of molecular transport in geological systems with shales of different TOC contents. The first scenario considers the progression of a diffusion front from a constant source (gas reservoir) into a homogeneous “semi-infinite” shale caprock over a period of 10 Ma. In the second example, gas diffusion across a 100 m caprock sequence is analyzed in terms of absolute quantities and isotope fractionation effects. The examples demonstrate that methane losses by molecular diffusion are small in comparison with the contents of commercial size gas accumulations. The degree of isotopic fractionation is related inversely to the quantity of diffused gas so that strong fractionation effects are only observed for relatively small portions of gas. The experimental data can be readily used in numerical basin analysis to examine the effects of diffusion-related isotopic fractionation on the composition of natural gas reservoirs.

Grain assemblages and strong diagenetic overprinting in siliceous mudrocks, Barnett Shale (Mississippian), Fort Worth Basin, Texas

Porosity, permeability, and total organic carbon (TOC) in a heterogeneous suite of 21 high-maturity samples (vitrinite reflectance 1.52–2.15%) from the Barnett Shale in the eastern Fort Worth Basin display few correlations with parameters of rock texture, fabric, and composition, these factors being mostly obscured by the effects of a protracted history of diagenesis. Diagenesis in these rocks includes mechanical and chemical modifications that occurred across a wide range of burial conditions. Compaction and cementation have mostly destroyed primary intergranular porosity. The porosity (average 5 vol. % by Gas Research Institute helium porosimetry) and pore size (8 nm median pore-throat diameter) are reduced to a degree such that pores are difficult to detect even by imaging Ar ion–milled surfaces with a field-emission scanning electron microscope. The existing porosity that can be imaged is mostly secondary and is localized dominantly within organic particulate debris and solid bitumen. The grain assemblage is highly modified by replacement. A weak pattern of correlation survives between bulk rock properties and the ratio of extrabasinal to intrabasinal sources of siliciclastic debris. Higher porosity, permeability, and TOC are observed in samples representing the extreme end members of mixing between extrabasinal siliciclastic sediment and intrabasinal-derived biosiliceous debris. Reservoir quality in these rocks is neither more strongly nor more simply related to variations in primary texture and composition because the interrelationships between texture and composition are complex and, importantly, the diagenetic overprint is too strong.

Origin and accumulation of carbon dioxide in the Huanghua depression, Bohai Bay Basin, China

The CO2 content in natural gas in the Huanghua depression, Bohai Bay Basin, China, is highly variable, ranging from 0.003 to 99.6%. Understanding the origin and distribution of the CO2 is important to assess risk prior to drilling. This study uses gas geochemistry to identify the origins of CO2 in the sedimentary basin and places these findings within a geologic context. Chemical compositions, , 3He/4He, and 40Ar/36Ar were measured for 50 gas samples collected from gas- and oil-producing wells located in different tectonic regions in the depression. From these analyses, we determined that the CO2 in the Huanghua depression originated from three sources: thermal decomposition of organic matter, thermal decomposition of carbonate minerals, and mantle degassing. Gases with low amounts (3%) of CO2 tend to be organogenic. This organogenic CO2 occurs in hydrocarbon accumulations and is characterized by values ranging from 20 to 10 and low 3He/4He (R/Ra 1, herein R and Ra represent the 3He/4He ratio of sample and air, respectively). Carbon dioxide originating from thermal carbonate decomposition occurs as a minor component (10%) in hydrocarbon gas accumulations and is characterized by a narrow range of (2 to +2) and R/Ra 1. Huanghua depression natural gases with CO2 content in excess of 15% resulted from mantle degassing and mainly occur at the intersection of faults. These gases have 3He/4He ratios in excess of atmospheric value (R/Ra 1) and ranging from 5 to 3. Volatiles from mantle degassing during the postmagmatic stage are the most likely major source for CO2 in these high-CO2-content reservoirs. Basement faults likely provide pathways for the upward migration of CO2-rich mantle fluids. Consequently, CO2-rich gas pools are locally concentrated in the Gangxi and Dazhongwang fault zones within the depression.

Pore and pore network evolution of Upper Cretaceous Boquillas (Eagle Ford–equivalent) mudrocks: Results from gold tube pyrolysis experiments

Low-maturity Boquillas Formation (Eagle Ford Formation–equivalent) organic-lean calcareous mudrock samples collected from outcrop were heated in gold tubes under confining pressure to investigate the evolution of organic matter (OM) pores and mineral pores. The majority of OM in the Boquillas samples was migrated petroleum (bitumen) based on evidence from geochemical analyses, solvent extraction, and scanning electron microscopy (SEM) petrography. The SEM images showed several diagenetic events—including framboidal pyrite precipitation and euhedral calcite, quartz, kaolinite, and chlorite cementation—that were all interpreted to have occurred prior to petroleum expulsion and pore-scale to bed-scale petroleum (bitumen) migration. Two major pore types were present prior to heating: primary mineral pores and modified mineral pores with migrated relic OM. From heating experiments, pores were found to be associated with stages of OM maturation. During the bitumen generation stage, modified mineral pores were dominant, and primary interparticle and intraparticle pores were present. During the oil generation stage, modified mineral pores with isopachous OM rim were observed to be the most abundant pore type. During the gas generation stage, both modified mineral pores and nanometer-sized spongy OM pores were predominant. We interpreted the occurrence of modified mineral pores to be the result of (1) oil and gas filled or partially filled voids that developed during petroleum migration and water expulsion; (2) voids after removing of oil, gas, and water during sample preparation; and (3) trapping of water molecules. The formation of these nanopores was interpreted to be related to gas generation and structural rearrangement of OM.

Origin and characterization of Eagle Ford pore networks in the south Texas Upper Cretaceous shelf

ABSTRACT Recent studies have shown that the loss of primary pores and the development of secondary pores in mudrocks are primarily controlled by burial diagenesis of the mineral matrix and thermal maturation of organic matter (OM). However, the lack of quantitative data on nanometer- to micrometer-scale rock properties has limited the ability to define and predict petrophysical properties and fluid flow in these fine-grained rocks. To upscale these rock properties, quantitative data are needed at multiple scales. Representative Eagle Ford Group samples were collected from continuous cores taken from two adjacent oil-producing wells in Karnes County, Texas, to investigate small-scale variations in mineralogy, diagenesis, and pore type. Point-count and pore-tracing methods were used to systematically quantify pore types and determine the size and shape of the identified pores. The two cores from the Eagle Ford are dominated by modified mineral pores, although secondary OM pores in migrated petroleum (bitumen) are also important. The mineral-pore network includes (1) primary mineral pores originally saturated with formation water and (2) modified mineral pores containing migrated petroleum (bitumen and/or residual oil). The OM-pore network includes (1) primary OM pores and (2) secondary OM pores including relatively large, less abundant OM bubble pores and relatively small, more abundant OM spongy pores. The abundance of OM spongy pores correlates positively with total-organic-carbon (TOC) content, and that of mineral pores weakly correlates with the volume of quartz plus feldspar. Studied samples have similar thermal maturities, although samples from one deeper core are slightly more mature than the other. Except for thermal maturation, the strong, micrometer-scale heterogeneity of rock components and properties (texture, fabric, mineralogy, and TOC) impacts the abundance, distribution, and type of pores. This micrometer-scale heterogeneity in porosity and pore networks would, in turn, significantly impact matrix permeability.

Pore types, pore-network analysis, and pore quantification of the lacustrine shale-hydrocarbon system in the Late Triassic Yanchang Formation in the southeastern Ordos Basin, China

AbstractContinental Upper Triassic Yanchang “black shales” in the southeastern Ordos Basin have been proven to be unconventional gas reservoirs. Organic-matter-lean and organic-matter-rich argillaceous mudstones form reservoirs that were deposited in a deeper water lacustrine setting during lake highstands. In the stratified lake, the bottom waters were dysaerobic to anoxic. This low-energy and low-oxygen lake-bottom setting allowed types II and III organic matter to accumulate. Interbedded with the argillaceous mudstones are argillaceous arkosic siltstones deposited by gravity-flow processes. Rock samples from the Yanchang Chang 7–9 members are very immature mineralogically. Mineral grains are predominantly composed of relatively equal portions of quartz and feldspar. The high clay-mineral content, generally greater than 40%, has promoted extensive compaction of the sediments, permitting the ductile material to deform and occlude interparticle pores. Furthermore, this high clay-mineral content does not f...

Regional assessment of CO2-solubility trapping potential: A case study of the coastal and offshore Texas Miocene interval

This study presents a regional assessment of CO2-solubility trapping potential (CSTP) in the Texas coastal and offshore Miocene interval, comprising lower, middle, and upper Miocene sandstone. Duans solubility model [Duan et al. Mar. Chem. 2006, 98, 131-139] was applied to estimate carbon content in brine saturated with CO2 at reservoir conditions. Three approaches (simple, coarse, and fine) were used to calculate the CSTP. The estimate of CSTP in the study area varies from 30 Gt to 167 Gt. Sensitivity analysis indicated that the CSTP in the study area is most sensitive to storage efficiency, porosity, and thickness and is least sensitive to background carbon content in brine. Comparison of CSTP in our study area with CSTP values for seven other saline aquifers reported in the literature showed that the theoretical estimate of CO2-solubility trapping potential (TECSTP) has a linear relationship with brine volume, regardless of brine salinity, temperature, and pressure. Although more validation is needed, this linear relationship may provide a quick estimate of CSTP in a saline aquifer. Results of laboratory experiments of brine-rock-CO2 interactions and the geochemical model suggest that, in the study area, enhancement of CSTP caused by interactions between brine and rocks is minor and the storage capacity of mineral trapping owing to mineral precipitation is relatively trivial.

Organic petrology of peak oil maturity Triassic Yanchang Formation lacustrine mudrocks, Ordos Basin, China

AbstractAn organic petrology evaluation and a determination of solid bitumen reflectance BRo were completed for organic-rich Triassic Yanchang Formation mudrocks (n=15) from the Ordos Basin, north-central China, as part of a larger investigation of “shale gas” resources. These data were integrated with information from Rock-Eval programmed pyrolysis to show that the samples are in the peak oil window of thermal maturity and that organic matter is dominated by solid bitumen with minor amounts of type III kerogen (vitrinite and inertinite) from vascular land plants. Describing a “kerogen type” for these rocks based strictly on parameters determined from programmed pyrolysis is misleading because the original organic matter has converted to hydrocarbons (present as solid bitumen), a large proportion of which may have been expelled into adjacent reservoir facies. However, based on the comparison with immature-early mature lacustrine mudrock (Garden Gulch Member of Green River Formation) and marine shale (Boqu...

Structural and hydrogeologic evolution of the Putumayo basin and adjacent fold-thrust belt, Colombia

This multidisciplinary study evaluates the structural and hydrogeologic evolution of Cretaceous-age reservoirs in the Putumayo basin, Colombia. We focused on the Eastern Cordillera fold-thrust belt along the southern Garzon Massif. Many important hydrocarbon accumulations occurred regionally along the proximal foreland basin and frontal fold-thrust belt defining the eastern margin of the northern Andes. To understand why recent Putumayo basin and adjacent thrust belt exploration has resulted in a wide range of oil quality and limited economic discoveries, we reconstructed the structural evolution, timing of oil migration, and timing of groundwater infiltration by (1) assessing regional trends in formation water, oil, and reservoir properties; (2) quantifying the timing of hydrocarbon generation and migration relative to trap formation using (a) two-dimensional (2-D) and three-dimensional seismic data to define and constrain a restorable balanced cross section from the Upper Magdalena Valley to the Putumayo foreland and (b) coupled one-dimensional thermal basin modeling; (3) evaluating the potential roles of Mesozoic extensional faulting and Paleogene shortening in the generation and preservation of structural traps; and (4) assessing groundwater influx from the modern foothills into the reservoir using a 2-D numerical groundwater flow model. We suggest that four-way closure is limited in the study area, where most foreland-verging structures create three-way fault closures that do not effectively trap light hydrocarbons. In addition, east-dipping structures and a relatively large reservoir outcrop area provide water infiltration pathways. Groundwater modeling suggests reservoirs were water washed by 2–200 million pore volumes since Andean uplift. Finally, average reservoir temperatures are <80°C (<176°F), which further facilitated biodegradation.