Tectonic-Lithofacies Paleogeography and Sizeable Reservoir Potential in the Early Cambrian Postrift Depressed Carbonate Ramp, Tarim Basin, NW China

Abstract

The carbonate ramps developed at the postrift depositional stage have become a major field of ultradeep hydrocarbon exploration. Taking the Lower Cambrian Xiaoerblak Formation in the Tarim Basin as an example, the paleotectonic framework and lithofacies paleogeography response, reservoir distribution, and internal architecture difference were systematically analysed based on the data of newly processed 42 2D seismic lines, 9 outcrops, and 22 wells. The results show that (1) the palaeotectonic framework with three palaeohighs and two topographic lows (abbreviated to “TPTT”) dominated the differentiation in the ramp system (association) at the Xiaoerblak depositional stage. High-energy grain shoals and mounds occurred around the palaeohighs, and the sediment composition was further complicated by ancient ocean currents. Three subtypes of ramp are recognized: the slope-plateau ramp dominated by grain shoal on the northern margin of the paleohigh of the southwestern Tarim, homocline ramp dominated by the mound-shoal complex on the Keping-Wensu area lower high, and isolated island type in the Lunnan-Yaha lower high. (2) Xiaoerblak reservoirs mainly occur in the high-energy facies belts in the middle ramps around the palaeohighs because original pores were effectively preserved in early dolomitization and reconstructed at the burial stage. Reservoirs are predicted to mainly distribute in the high-energy mound-shoal zones around three palaeohighs (low uplifts) with an area of 9×104 km2. Different ramp systems show different reservoir properties. There are mainly type II reservoirs (porosity range of 1.8-4.5%, permeability range of 0.5-10×10−3 μm2) with sustained distribution and the largest size in the grain shoals at the slope-plateau ramp, which may be the potential favorable hydrocarbon exploration zone. Based on the example anatomy, the differentiation in the carbonate ramp systems forming at the postrift subsidence stage was structurally dominated. This means that it is crucial to deepen the understanding of the presedimentation palaeotectonic framework for the prediction of promising reservoirs and assessment of reservoir properties.

Facies, Depositional Environment and Reservoir Quality of an Early Cambrian Carbonate Ramp in the Tarim Basin, NW China

“…In the late Ediacaran, the Tarim Basin entered the rift-depression transition stage of the ancient rift system from the rift stage owing to mantle bulge, crustal attenuation or extension (Figure 1b). In addition to local synsedimentary faults, the basin was dominated by thermal subsidence and demonstrated differential characteristics, namely "uplift in the south and platform in the north, high in the south and low in the north" [14,25]. At the end of Ediacaran, the Cryogenian-Ediacaran Formations in the whole basin were denuded to varying degrees, while a regional large-scale unconformity between Ediacaran Formations and Cambrian Formations was generated owing to the Keping movement [26].…”

“…The Cryogenian-Middle Cambrian sedimentary succession of the Tarim Basin has been extensively studied, mostly through drilling data for hydrocarbon exploration [12,14,19,25,28]. The Cryogenian-Ediacaran sedimentary succession in the Tarim Basin was deposited with extremely thick coarse clastic deposits interbedded with mudstone and carbonate rocks, multiple sets of volcanic extrusive rocks, intrusive rocks and four sets of lodgement till deposits.…”

“…For example, Wells YL6, XH1, CT1 and HT2 failed successively (Figure 1a), calling into question the exploration direction and potential of deep to ultradeep carbonate rocks to a certain extent. The major theoretical and technical bottleneck is identifying whether to detect and finely describe the deep to ultradeep largescale high-quality reservoirs [11,14,15]. Global case studies show that the distribution of large-scale carbonate reservoirs is mostly facies-controlled [6,10,16,17].…”

“…Economic interest has driven increased research into the sedimentary facies of the Early Cambrian Xiaoerbulake Formation in the Tarim Basin. Researchers have proposed that the Xiaoerbulake Formation belongs to the sedimentary facies zone of ramp, ramptidal flat, restricted platform, semi-evaporative platform, open platform margin or slopebasin [14,[18][19][20][21]. However, previous studies mainly focused on limited well logging and cutting data and low-resolution 2D and 3D seismic data, but did not thoroughly address with the facies and facies associations of the Early Cambrian Xiaoerbulake Formation carbonate ramp.…”

Geochemical Characteristics and Their Geological Significance of Lower Cambrian Xiaoerblak Formation in Northwestern Tarim Basin, China

“…In the Early Cambrian, the Tarim Basin was in a rampshelf sedimentary system during the deposition of the Yuertus Formation, and formed high-quality source rocks after being transformed by rapid transgression [27]. Later, the whole basin stayed in a stage of slow regression, and the paleostructural pattern consisting of three uplifts and two depressions controlled the post-rift subsidence of the carbonate ramp system during the deposition of Xiaoerblak [28]. As the paleo-climate became hot and dry and the platform evolved with a barrel-shaped structure, thick evaporites were widely developed in the Middle Cambrian, forming the regional high-quality caprocks [29].…”