Selective dissolution of alkali feldspars and its effect on Lower Triassic sandy conglomerate reservoirs in the Junggar Basin, northwestern China

Abstract

Feldspar dissolution resulting from fluid–rock reactions in clastic rocks is common in petroliferous sedimentary basins and significantly affects the quality of reservoirs. The objective of this study is a case study of this phenomenon in the sandy conglomerate reservoirs, based on the Lower Triassic Baikouquan Formation in the Mahu Sag at the northwestern margin of the Junggar Basin in northwestern China, which has obtained exploration breakthroughs recently. In these rocks, orthoclase has undergone dissolution and the dissolution intensity decreases gradually from the basal member (T1b1) to the upper member (T1b3) of the formation, whereas albite remained stable or has even overgrown crystals, displaying a selective dissolution of alkali feldspars. This is interpreted to be mainly controlled by two factors. First, illitization of mixed‐layer illite/smectite in the sandy conglomerates consumed K, thereby increasing Na/K ratios in the formation waters and leading to orthoclase dissolution and albite overgrowths on perthite and microcline. Second, the charge of acidic, hydrocarbon‐related fluids further promoted orthoclase dissolution. These relationships are confirmed by the fact that hydrocarbon charging intensity shows a gradual upward reduction from member T1b1 to T1b3, as does the scale of orthoclase dissolution. This selective dissolution resulted from the influence of different mineral components on diagenesis, the acidic nature of reservoir fluids, and the extent of water–rock interaction. The dissolution significantly improved porosity and permeability of the reservoirs by generating substantial secondary porosity. The results indicate a favorable exploration prospect located in the up‐dip area of the Ma 18 and Aihu 2 wells on the western slope of the Mahu Sag.

Feldspar Dissolution and Its Influence on Reservoirs: A Case Study of the Lower Triassic Baikouquan Formation in the Northwest Margin of the Junggar Basin, China

“…By the Triassic, the entire basin had evolved into an intracratonic basin, with sedimentation occurring on a stable basement. In the Late Triassic, the margins of the basin were still compressed [30]. The Baikouquan Formation is a gentle monocline structure that tilts to the southeast [38].…”

“…In the study area, the microfracture is less developed which mainly formed at the edge of the gravel and the pore formed by grain dissolution is the main storage space in the reservoir. Feldspar dissolution pores account for the highest percentage of all the secondary pores [30] Feldspar grains that have a honeycomb-like texture in porous conglomerate reservoirs usually contain significant (Figure 8(a)) [30]. The quantitative porosity analysis of the thin sections shows that the porosity formed by feldspar dissolution can reach up to 6% and feldspar dissolution increases with increasing depth (Figure 10(a)).…”

“…The data show that the salinity of the samples near the fault is relatively low (all lower than 6000 mg/L) and the fault can provide an effective channel for fluid migration, which highly impacts the feldspar dissolution. In addition, the organic acids generated by the source rocks of the Fengcheng Formation migrate along faults to reservoirs, which has an important effect on the feldspar dissolution [30].…”

“…Recently, continuous discoveries have been made and the proven reserves of the basin have reached 1.83 million tons, which indicates that the Baikouquan Formation is a good exploration prospect. In the study area, because of the high mud content and the poor original reservoir properties, feldspar dissolution is one of the key factors that determine the formation of high-quality reservoirs [30]. Therefore, this paper carries out a large number of core observations, intensive and representative sample collection, advanced research methods, and experimental means to analyse the influence of feldspar dissolution, its by-products on the reservoir, and provide a reliable basis for the future exploration of favourable reservoirs.…”

An overview of types and characterization of hot fluids associated with reservoir formation in petroliferous basins

“…The Triassic Baikouquan Formation is an important conglomerate reservoir in the Mahu Sag of the Junggar Basin (Kang et al, 2018). Calcite cements in the conglomerates are characterized by extremely negative d 13 C values as low as À70& (V-PDB; unpublished data).…”