Facies controls on the distribution of diagenetic alterations in fan delta deposits: a case study from the Lower Cretaceous sandstone, Chagan Sag, Inner Mongolia, China

Abstract

By integrating diagenesis and sedimentary facies, the distribution of diagenetic alterations and their impacts on reservoir quality were investigated within a lacustrine fan delta depositional environment in the Lower Cretaceous of the Western sub‐sag in the Chagan Sag, Yin‐E Basin, northern China. Core observation and analyses of petrography and geochemistry of the sandstones revealed that the eogenetic alterations display spatial and temporal distribution patterns associated with sedimentary facies, including distributary channels of a fan delta front (DC), distributary bays (DB) and sheet sands (SS) and mouth bar (MB) (SS and MB are collectively referred to as S). Percolation of meteoric waters occurring in high permeability DC sandstones resulted in leaching of feldspar, the formation of kaolinite, as well as mechanical infiltrated clays around detrital grains. Conversely, the DB deposits containing abundant ductile lithic fragments were subjected to mechanical compaction and thus the development of a pseudomatrix. During a long burial residence time (the Early Cretaceous period) in a lacustrine environment, abundant early carbonate cements were precipitated, especially in the distal S sandstones. The sandstones lacking eogenetic cements were subjected to stronger mechanical compaction. Additionally, eogenetic alterations have an important impact on the distribution of the mesogenetic alterations. Sandstones containing few eogenetic cements or thin or discontinuous infiltrated clay rims around the detrital grains were subjected to quartz cementation. However, during the hydrocarbon generation stage, the most efficient percolation of meteoric waters and organic acid dissolved feldspar minerals and carbonate cements, resulted in DC sandstones having more intragranular and intergranular porosity. Owing to high matrix contents and early calcite cement contents, the DB and S sandstones had lower permeability and were rarely dissolved. During the late mesogenetic stage, late carbonate cements occurred in all sedimentary facies and iron and magnesium ions were released from the transformation from kaolinite to illite or to chlorite. The results from this study show possible diagenetic evolutionary pathways in the reservoir sandstones within the fan delta depositional environment, which in turn provides some insights into the controls on reservoir potential. Copyright © 2016 John Wiley & Sons, Ltd.

Diagenesis and reservoir quality of sandstones in the Eocene Shahejie Formation, Raoyang Sag (Jizhong Depression), Bohai Bay Basin, China

“…For example, the poorly sorted, fine‐grained samples or samples rich in detrital clays are observed to have experienced more intense mechanical compaction. Line and concavo–convex contacts dominate the Diagenetic Facies B, indicating the reorientation of detrital grains due to mechanical compaction in eodiagenesis, as found by Wei et al () in their study of Lower Cretaceous sandstone in the Chagan Sag of the Erlian Basin in China. Subjected to grain rearrangement, no pores (primary pores or secondary pores) can be found in Diagenetic Facies B under thin‐section observations (Figures and ).…”

“…The texture of illite and mixed‐layer illite/smectite, for example, fibrous, honeycomb is an indicator of significant reduction in permeability. The honeycomb‐like illite was generally formed by the transformation of authigenic smectite with potassium sourced from dissolution or alteration of K‐feldspars (Figure ), as found by Lanson et al (), Worden and Morad (), Higgs et al (), Khalifa and Morad (), and Wei et al (). In contrast, illitization of smectite or kaolinite is temperature dependent, occurring at temperatures of higher than 70°C and being pervasive at temperature higher than 130°C (Figure ).…”

“…Actually, the low‐permeability sandstones are generally characteristic of a complex pore system (Higgs, Zwingmann, Reyes, & Funnell, ; Lai, Wang, Fan, et al, ; Lai & Wang, ). Diagenesis, which involves mechanical compaction and mineral precipitation and dissolution, can exert a significant impact on porosity and permeability (Bjørlykke & Jahren, ; Lai, Wang, Cai, et al, ; Lai, Wang, Ran, Zhou, & Cui, ; Morad, Al‐Ramadan, Ketzer, & DeRos, ; Wang, Chang, Yin, Li, & Song, ; Wei et al, ; Wu, Liu, Du, Jiang, & Cheng, ; Yuan, et al, ; Zou et al, ), while complicated diagenesis brings more challenges to reservoir quality prediction for these deeply buried sandstones (Yuan, Cao, et al, ). Therefore, understanding the impact of diagenesis on reservoir quality is critical to hydrocarbon exploration and production (Khalifa & Morada, ).…”

The sequence stratigraphy and depositional characteristics of fan‐delta complexes in the Upper Bayingebi Member (Lower Cretaceous) in Chagan Sag, Inner Mongolia, China

“…Generalized geological map (a) and structural profile (b) of Chagan Sag showing tectonic units and study area (modified from Wei et al, ; Wei et al, ; ; Yang, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…The current sedimentological understanding of the Chagan Sag is quite insufficient due to inefficient public transportation in the past. In addition, previous studies have merely unravelled the depositional characteristics of predominant members (Li, Chen, Fan, Wang, & Feng, ; Yang et al, 2013a; Yang et al, 2013b; Wei et al, ; ), with the impact of the depositional process and sequence architecture being consistently neglected when considering differences between the ramp‐type fan delta and steep‐style fan delta in the study area.…”