Controls on pore types and pore-size distribution in the Upper Triassic Yanchang Formation, Ordos Basin, China: Implications for pore-evolution models of lacustrine mudrocks

Abstract

Our main objectives are to (1) learn if pore-evolution models developed from marine mudrocks can be directly applied to lacustrine mudrocks, (2) investigate what controls the different pore types and sizes of Chang 7 organic matter (OM)-rich argillaceous mudstones of the Upper Triassic Yanchang Formation, and (3) 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 ([Formula: see text]) is higher than those from the two adjacent cored wells ([Formula: see text]). 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 (sorting and grain size) 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.

Effect of organic matter composition on source rock porosity during confined anhydrous thermal maturation: Example of Kimmeridge-clay mudstones

“…Some of them appear to engulf pyrite grains (Fig.11A). As suggested in previous studies (Guo et al, 2018;Ko et al, 2017Ko et al, , 2018Loucks et al, 2012;Loucks and Reed, 2014;Milliken et al, 2013) these particles may correspond to migrated material resulting from kerogen conversion (bitumen, oil or preoil gel-like solid bitumen) probably mixed with amorphous kerogen particles. Locally, these OM particles exhibit a porous granular texture (Figs.10D, 11C).…”

“…For a given maturity, Ko et al (2018) showed that two mudstones characterized by differences in maceral composition can exhibit very different pore evolution models. Other non-maturity-related factors appear to have locally a significant effect on porosity such as the organic richness (Milliken et al, 2013;, the type or the original maceral composition of kerogen and its ability to produce oil (Cardott et al, 2015;Cardott and Curtis, 2018;Curtis et al, 2012;Guo et al, 2018;Ko et al, 2017Ko et al, , 2018Loucks et al, 2012;Mastalerz et al, 2013), the nature of the rock framework and the matrix porosity (Ko et al, 2017;Milliken et al, 2013;Pan et al, 2015). However, isolating the influence of these factors on porosity during burial and thermal maturation turns out to be a complex task.…”

A review of gas transport and adsorption mechanisms in two‐component methane‐carbon dioxide system

“…At low maturity, compaction plays a dominant role. At this stage, organic pores are not developed, and mesopores (2‐50 nm) and macropores (>50 nm) in shale are decreasing . When the maturity is near the oil window, the original pores are blocked because of the large amount of liquid hydrocarbons generated.…”

“…At this stage, organic pores are not developed, and mesopores (2-50 nm) and macropores (>50 nm) in shale are decreasing. 93 When the maturity is near the oil window, the original pores are blocked because of the large amount of liquid hydrocarbons generated. At this stage, micropores (<2 nm), mesopores, and macropores are continuously decreasing.…”

Fractal Characteristics of Pores in Taiyuan Formation Shale From Hedong Coal Field, China

“…Both types of iron ore-related pores have relatively good connectivity which may provide narrow channels and permeability pathways for gas flow. 15,32,37 …”