Paleokarst of the Lianglitage Formation related to tectonic unconformity at the top of the Ordovician in the eastern Tazhong Uplift, Tarim Basin, NW China

Abstract

The Lianglitage Formation (also named Lianglitag Formation) is an important hydrocarbon reservoir in the Tarim Basin; the reef and shoal facies of this formation have previously been extensively investigated. The paleokarst of the formation related to a tectonic unconformity at the top of the Ordovician (Tg5) has, however, not been widely investigated. In this paper, we undertake a comprehensive study of the paleokarst by an integrated analysis of seismic data, well‐logging data, and core data. Several geomorphologic units that formed at the end of the Ordovician period are identified, including a fault‐thrust uplift high, inter‐uplifts sag, uplift platform, and fault‐thrust anticline; these units are controlled by west–east and northwest–southeast striking faults. The thickness of erosion is estimated to be 200 to 700 meters. The paleokarst is characterized by large and small caves, dense vugs, and dissolution fractures. Large caves predominantly developed in exposed areas and on the slopes. Dense vugs and small caves, features that contribute to premium hydrocarbon reservoirs, are generally tens of meters away from the unconformity, whereas dissolution fractures, predominantly filled by mud and silts, are situated in close proximity to the unconformity and faults. Four karst zones can be identified from vertical profiles: weathered crust (1–2 m thick), vertical infiltration zone (tens of meters thick), phreatic zone (tens to hundreds of meters thick), and a bottom zone that is largely unaffected by karstification. The karst of the Lianglitage Formation at the end of the Ordovician was mainly controlled by tectonic movement, paleogeomorphology, lithofacies, faults, and fractures. The karstification process is summarized in a karst model.

Diagenesis Sequence and Hydrocarbon Accumulation Period of the Ordovician Reservoir in Well Tashen-6, Tahe Oilfield, Tarim Basin, NW China

“…The Akekule area has experienced multistage tectonic movement and transformation events that played an important role in controlling the structural characteristics of the Tahe Oilfield, as well as hydrocarbon migration and accumulation. , Generally, the main tectonic movements of Caledonian, early Hercynian, and late Hercynian ages controlled the development of reservoir space and the channel and direction of hydrocarbon migration, while the Indosinian–Yanshanian and late Himalayan were important periods for oil and gas refilling, adjustment and finalization. − That is, the occasional tectonic movements and weathering and denudation during the middle Caledonian provided oil and gas reservoir space and were dominated by karst fracture holes or karst caves in the Tahe area . The strong tectonic uplift during the early Hercynian also provided large-scale accumulation space for hydrocarbons.…”

Microfacies and Reservoir Space Characteristics of Carbonate Reservoir of the Ordovician Yingshan Formation in Northern Slope of Tazhong Uplift, Tarim Basin

“…In the middle of the Caledonian movement, the Tazhong belt was denuded by uplift and developed large-scale angular unconformity, resulting in a large-scale deletion of the upper Yingshan Formation and the Tumuxiuke Formation in the Tazhong area. It was not until the early Late Ordovician period that it began to redeposition the Lianglitage Formation, with which was inconformed at an angle [12][13][14].…”

Paleokras w utworach górnej jury podłoża zapadliska przedkarpackiego i jego znaczenie złożowe

“…Nie ma powodu, by te formy nie występowały również w formie kopalnej, jako tzw. paleokras, czyli kras kopalny, na co są zresztą liczne dowody zarówno w Polsce, jak i na świecie (Zhang i Liu, 2009;Szulc et al, 2015;Wójcik et al, 2015;Bardziński i Kurowska, 2017;Cao et al, 2017;Gao et al, 2018). Rejon przedgórza karpackiego po osadzeniu wapieni górnojurajskich był wielokrotnie poddany procesom erozyjnym w warunkach wietrzenia lądowego, m.in.…”

“…O tym, jak istotne znaczenie może mieć paleokras dla złóż węglowodorów, może świadczyć przykład Chin, gdzie odkryto wiele złóż ropy naftowej i gazu właśnie w paleokrasie (Zhang i Liu, 2009;Lin et al, 2012;Sun et al, 2013;Jiang et al, 2015;Dai et al, 2017;Tian et al, 2017;Gao et al, 2018). Na przykład w basenie Tarim, położonym w NW Chinach, zasoby ropy i gazu występujące w strukturach krasowych oceniane są na 113,55 × 10 8 t ropy naftowej i 13,183 × 10 12 m 3 gazu.…”