The role of the primary detrital grain assemblage as a control on diagenetic pathways is reasonably well-understood in sandstones and limestones, but less so in mudrocks. We have documented diagenesis in mudstones from the Triassic Yanchang Formation that are dominated by grains derived from outside the basin of deposition. Major extrabasinal grains are K-rich clay, quartz, plagioclase, K-feldspar, lithic fragments, and micas. In terms of the quartz-feldspar-lithic grain compositions, the silt fraction in these samples is classified as arkose. Grains of (...) intrabasinal derivation include particulate organic matter, phosphatic debris, and rare carbonate allochems. The principal chemical diagenetic components in these mudrocks have strongly localized spatial distributions at micrometer to centimeter scales. Chemical diagenetic components include cone-in-cone structures, replacements of detrital feldspar, pore-filling precipitates within anomalously large pores, pore-filling solid hydrocarbon, and very minor quartz overgrowths associated with local packing flaws around silt-size detrital quartz grains. Matrix-dispersed intergranular cementation, as observed in well-known organic-rich marine mudstones, such as the Barnett Shale and the Eagle Ford Formation, is not observed in Yanchang Formation lacustrine mudstones. The authigenic features present are consistent with the thermal maturity of the units and are broadly similar to features observed in other mudstones that contain grain assemblages dominated by particles of extrabasinal derivation. The low porosity and the absence of significant amounts of intergranular cement indicate that compactional porosity loss and in-filling by migrated solid hydrocarbon were the major causes of porosity decline during diagenesis of Yanchang Formation mudrocks. Although the mudstones of the Yanchang Formation have a relatively high content of organic carbon and serve as source rocks in the Ordos Basin, the depositional grain assemblage is not conducive to creation of porosity, permeability, and mechanical properties that would make these mudrocks effective unconventional reservoirs. (shrink)

Our main objectives are to learn if pore-evolution models developed from marine mudrocks can be directly applied to lacustrine mudrocks, investigate what controls the different pore types and sizes of Chang 7 organic matter -rich argillaceous mudstones of the Upper Triassic Yanchang Formation, and describe the texture, fabric, mineralogy, and thermal maturity variation in the Chang 7 mudstones. Lacustrine mudstones from nine cored wells along a depositional dip in the southeastern Ordos Basin, China, were investigated. Helium porosimetry, nitrogen adsorption, and (...) field-emission scanning electron microscopy of Ar-ion milled samples were applied. Measured average total porosity of samples from a proximal to distal transect is higher than those from the two adjacent cored wells. This difference in porosity partly caused by differences in the clay mineral content implies that in the fluvial-deltaic-lacustrine depositional environment, reservoir quality can vary significantly in a short distance. Owing to the uneven distribution of the sample set from proximal to distal area, we mainly evaluate variations in the proximal setting. Results from nitrogen-gas adsorption experiments show that there are four distinct patterns of pore-size distribution within the Chang 7 member of the Yanchang Formation with no particular correlation with mineralogical composition and thermal maturity. The pore network within Chang 7 mudstones is dominated by OM-hosted pores, with a lesser abundance of interparticle and intraparticle pores. The size distribution of mineral-hosted pores within these mudstones is found to be closely related to the rock texture and fabric. Mudstones with well-sorted grains and a higher percentage of coarser grains have more abundant mineral pores. The sizes of OM-hosted pores in these compaction-dominated lacustrine mudstones were one to two orders of magnitude smaller than those in the marine mudstones that display abundant early cementation. (shrink)

A chemostratigraphic study of a 300 m long core recovered from the southeastern central Ordos depocenter reveals thick intervals of fine-grained, organic-rich lacustrine strata, interpreted to represent deepwater deposition under meromictic conditions during lake highstand phases, interspersed with thick intervals of arkosic sandstones, reflective of fluvio-deltaic deposition during lake lowstand phases. Along with elevated concentrations of %Al, traditionally a proxy for clay content, maximum total-organic-carbon values in the deepwater lacustrine facies reach 8%, with average values of approximately 3%. The fine-grained, (...) organic-rich facies is also characterized by elevated S and As concentrations, both proxies for pyrite, an indicator of more stagnant, reducing conditions in the hypolimnion during lake highstand phases. Enrichment factors for redox-sensitive trace elements are not significantly elevated throughout the TOC-rich intervals, but they are punctuated in thinner intervals. Punctuated enrichments in RSTEs reflect episodes of enhanced suboxia/anoxia in the hypolimnion during lake highstand phases resulting from sustained meromixis. Although dramatic shifts between fluvio-deltaic and deepwater lacustrine deposition are recorded in the Yanchang strata, no evidence indicates that the lacustrine system ever built up a significant salt concentration, suggesting that an outlet was maintained throughout much of the depositional history. A chemofacies framework for the Yanchang Formation is developed based on hierarchical cluster analysis and ranking of major element chemostratigraphic results. The outcome of chemofacies analysis is similar to the lithofacies analysis, subdividing the stratigraphic record of calcite-cemented sandstone/siltstone lithologies and silty to finer grained mudstone lithologies based on changes in elemental concentrations that equate to shifts in the relative abundance of mineral contents. (shrink)

The Late Triassic Yanchang Formation is a major target of drilling for hydrocarbons in the Ordos Basin. Although most of the early focus on this thick succession of lacustrine rocks has been the dominant deltaic sandstones and siltstones, which act as local reservoirs of oil and gas, more recent consideration has been given to the organic-rich mudstone source rocks. We used modern chemostratigraphic analysis to define vertical facies successions in two closely spaced cores through the Chang 7 Member, the primary (...) source rock for the Yanchang hydrocarbon system. We used integrated high-resolution X-ray fluorescence and X-ray diffraction measurements to define four dominant facies. Variations in stable carbon isotopes mimic facies stacking patterns, suggesting that terrigenous organic matter is associated with the arkoses and sandstones, whereas aquatic organic matter is dominant in the mudstones. Facies stacking patterns define three major depositional cycles and parts of two others, each defined by basal mudstone facies that document basin flooding and deepening. Unconfined compressive strength measurements correlate with clay mineral abundance and organic matter. Comparisons of core attributes with wireline logs indicate that although general variations in clay mineral volumes can be discerned from gamma-ray logs, organic-matter distribution is best defined with density or resistivity logs. These findings, especially those established between the core and log data, provide a powerful linkage between larger scale facies patterns and smaller scale studies of key reservoir attributes, such as pore systems, mineralogy, diagenesis, rock mechanics, hydrocarbon saturation, porosity and permeability, and flow parameters. This first application of modern chemostratigraphic techniques to the Yanchang Fm reveals the great promise of applying these methods to better understand the complex facies patterns that define this lacustrine basin and the variations in key reservoir properties that each facies displays. (shrink)

Understanding the structure of the pore throat is an important task for tight reservoir development. The common methods of studying the pore-throat structure include pressure-controlled mercury injection, rate-controlled mercury injection, X-ray computer tomography, and scanning electron microscopy. We have used those four methods to understand the pore-throat structures of three main lithologies in the Lucaogou Formation, Jimsar Sag, China. The PMI results revealed that the pore-throat size of all of the lithologies ranges from 0.013 to [Formula: see text]. The RMI (...) testing indicates that the different lithologies within the Lucaogou Formation illustrate a similar pore structure but a different throat structure. The testing of PMI, RMI, and CT together indicated that the size of the throat is the main factor affecting the permeability. The experiments’ results indicated that the pore-throat structures of the tuffaceous siltstone, dolomitic siltstone, and silty dolomicrite are better than those of the other lithologies. (shrink)

Understanding the structure of the pore throat is an important task for tight reservoir development. The common methods of studying the pore-throat structure include pressure-controlled mercury injection, rate-controlled mercury injection, X-ray computer tomography, and scanning electron microscopy. We have used those four methods to understand the pore-throat structures of three main lithologies in the Lucaogou Formation, Jimsar Sag, China. The PMI results revealed that the pore-throat size of all of the lithologies ranges from 0.013 to [Formula: see text]. The RMI (...) testing indicates that the different lithologies within the Lucaogou Formation illustrate a similar pore structure but a different throat structure. The testing of PMI, RMI, and CT together indicated that the size of the throat is the main factor affecting the permeability. The experiments’ results indicated that the pore-throat structures of the tuffaceous siltstone, dolomitic siltstone, and silty dolomicrite are better than those of the other lithologies. (shrink)