Discovery of syngenetic and eogenetic karsts in the Middle Ordovician gypsum-bearing dolomites of the eastern Ordos Basin (central China) and their heterogeneous impact on reservoir quality

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

“…The higher reservoir capacity (as shown by porosity and permeability measurements) in the northern karst slope than in the southern karst slope could be due to the differential dissolution‐filling effects of groundwater under the control of karst paleogeomorphology . 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 .…”

“…Six sublayers are further divided within Ma 5 1+2 , respectively, referred to as Ma 5 2 2 , Ma 5 2 1 , Ma 5 1 4 , Ma 5 1 3 , Ma 5 1 2 , and Ma 5 1 1 (Figure D). During deposition of the Ma 5 member, the basin was in a restricted‐evaporative epicontinental sea carbonate‐platform environment with high salinity, and the main reservoir was composed of gypsiferous moldic pore‐bearing micritic dolomites . During deposition of the Ma 6 member, the sedimentary environment gradually transitioned to open, with micritic dolomitic limestones as the main lithology, and there were no evaporates formed as in Ma 5.…”

“…During the Late Ordovician to Early Carboniferous, the Caledonian Orogeny uplifted the North China Platform above sea level . The Ordos Basin and its periphery were characterized by a series of uplifts and depressions controlled mainly by the Yimeng Paleo‐uplift in the north and the Central Paleo‐uplift in the center (Figure B).…”

“…However, some exploration wells in the eastern part of the basin, such as wells T1 and Zh1 (Figure , have since been shown to be highly productive. Moreover, further drillcore data and associated exploration have shown that the intensity of karst development in the eastern part of the basin is similar to that in the central part . All of these findings suggest that the current understanding of the karstic aspects of the Ordos Basin may not be fully accurate.…”

“…An extended and widespread hiatus lasting for up to 120 Myr during the Middle Ordovician to early Carboniferous in the North China Craton (NCC) corresponded to an important geological event, that is, Caledonian Movement, which led to a change from marine to transitional facies and eventually terrestrial facies. The Ordos Basin in the southwestern NCC recorded this process and therefore provides an opportunity to investigate the regional differential uplift pattern at that time, during which the extensive paleokarst unconformity and its paleogeomorphology had profound influences on the paleogeography and diagenesis of the corresponding strata . On the one hand, extensive and long‐term meteoric water karstification transformed the Ordovician Majiagou Formation carbonate rocks, contributing to the formation of the Jingbian gas field in the central part of the basin .…”