Petrography and facies distribution of Middle Ordovician Ma 5^1 + 2 tight dolomite reservoirs in the Ordos Basin, Central China

Abstract

The Middle Ordovician Majiagou Formation in the eastern Ordos Basin, central China, is an important area in the exploration for tight carbonate gas, especially within weathering crust layers in the first and second submembers of the fifth member of the formation (herein referred to as Ma 5 1 þ 2). However, karstification prevents a clear understanding of the petrological characteristics and facies distribution of these layers, which hinders exploration. Based on cores, thin sections, and cathodoluminescence analysis, we investigate the petrological characteristics of Ma 5 1 þ 2 , determine the nature of lateral lithological variations in the eastern and central parts of the Ordos Basin, and constrain facies distribution in the region. In addition to karst breccias with unrecognizable parent rocks, Ma 5 1 þ 2 comprises four lithologies: gypsum/halite mold-bearing micritic dolomite, micritic dolomite, grain dolomite, and microbial dolomite. We recognize three main sedimentary subfacies: restricted lagoon, grain shoal, and mound-shoal complex. Ma 5 1 þ 2 records a complete transgression-regression cycle. The Ma 5 2 2 layer was deposited during a transgression associated with enhanced water circulation and abundant mound-shoal complexes, for which their frequency is positively correlated with the thickness of the unit. The Ma 5 1 2 layer and overlying deposits correspond to a regression cycle, and the abundance of mound-shoal complexes in these units is negatively correlated with layer thickness. The Ma 5 1 3 period represents the timing of maximum regression, when a gypsum-bearing dolomitic lagoon was dominant, associated with a restricted water body. The overall facies

Reconstructing large‐scale karst paleogeomorphology at the top of the Ordovician in the Ordos Basin, China: Control on natural gas accumulation and paleogeographic implications

“…Previous studies have highlighted the formation of moldic pores by selective fabric dissolution resulting from syngenetic karst during the deposition of Ma 5 1+2 and argued that this process was the key to reservoir formation in karstic materials . However, there was no significant regional difference across the study area in terms of karstification during the deposition of Ma 5 1+2 . Alternatively, the regional differences in reservoir capacity could be explained by variation in Caledonian karstification, which was controlled largely by karst paleogeomorphology.…”

Controlling Factors of Meteoric Diagenesis in Karst Reservoirs: An Example from the Majiagou Formation, Ordos Basin, China

“…C and O isotopes showed variations in different lithologic samples (Table 3). Except for one sample (N3-11, −5.2‰), the δ 13 C contents of the matrix were concentrated in -2.8‰~+0.2‰, with an average of -1.0‰; and the δ 18 O contents were −9.8‰~−5.2‰, with an average of −7.1‰ (the δ 18 O content of N3-11 was −8.3‰). The δ 13 C and δ 18 O contents of…”

“…Four representative thin sections were selected for in situ major, trace, and rare earth element analysis. Powdered samples were selected for geochemical analyses (e.g., δ 13 C, δ 18 O, and 87 Sr/ 86 Sr), using a micro-mill with a drilling bit diameter of 0.25 mm. To distinguish between matrix and cement, a total of 20 samples were collected for C and O isotope analysis, and 16 samples were collected for Sr isotope analysis.…”

“…carbonate reservoirs have their own unique attributes, because they are composed of evaporites and carbonate rock assemblages under the influence of paleogeographical location and paleoclimate [15]. There are large-scale micrite dolomite reservoirs containing irregular-circular gypsum mold pores that are rarely found elsewhere [16][17][18]; these gypsum mold pores are filled with calcite cement or residual gypsum minerals [19][20][21]. These coarse-grained secondary calcites originated from meteoric freshwater during epigenetic diagenesis [13,22].…”

Dolomitization of the Ordovician subsalt Majiagou Formation in the central Ordos Basin, China: fluid origins and dolomites evolution

“…During the Middle Ordovician, the study area was a restricted-evaporatic carbonate platform, in which the circulation of seawater was greatly restricted (Chen et al 2018;Liu et al 2019). In such a geographic setting, a warm, semiarid to arid climate could have enhanced evaporation and resulted in elevated salinity of the seawater.…”