A Third‐order Unconformity Within Lower Ordovician Carbonates in the Tarim Basin, Nw China: Implications for Reservoir Development

Abstract

This paper presents outcrop, petrographic, geochemical, well log and seismic data which together characterise the third‐order T78 unconformity located between the carbonate‐dominated Lower Ordovician Penglaiba and Yingshan Formations in the Tarim Basin, NW China. Unconformities in Lower Palaeozoic carbonates in this basin are of increasing interest because major reserves of hydrocarbons have recently been discovered at the North Slope field (> 1000 × 106 brls oil and ∼ 3050× 108 m3 gas). The reservoir here consists of karstified Lower Ordovician carbonates bounded by a third‐order unconformity. The T78 unconformity in Tarim Basin represents a short‐term exposure surface (< 1 Ma) controlled both by sea‐level changes and by palaeogeographic location within the basin, and the intensity of karstification varies laterally. The unconformity has had a major influence on porosity development in the underlying Penglaiba Formation carbonates. At two measured outcrop sections at the NW basin margin (Penglaiba and Shuinichang), dissolution porosity was observed in karstified and dolomitised carbonates below the T78 unconformity surface. A seismic profile shows the presence of reflection anomalies below the unconformity which are interpreted as karst‐related palaeo‐caverns. Geochemical data indicate that the T78 unconformity is associated with anomalies in stable isotope ratios and in heavy mineral and trace element profiles. Thus there are negative excursions in δ13C and δ18O ratios within the carbonate rocks immediately below the unconformity surface. Similarly, concentrations of major and trace elements such as Li, K, Ti, Rb, Th, Sr, V and Ni are significantly reduced in the underlying carbonates, while there is an anomalously high content of haematite‐limonite.

New Insight into the Characteristics of Tight Carbonate based on Nuclear Magnetic Resonance

“…Ten samples were collected from the NYG (Nanyigou) outcrop in the northwestern margin of the Tarim Basin. Due to the similar strata, sedimentary characteristics, and structural evolution, the Yingshan Formation and the Yijianfang Formation exposed at the NYG outcrop have been considered to be analogues of the subsurface reservoirs in the Tahe area . The Lower Ordovician Yingshan Formation is divided into two parts by the unconformity (T 7 6 ).…”

“…The permeability of the cored plug samples was measured using a helium pulse-decay permeameter (NDP605) under an effective stress of 800 psi. In order to measure the distribution of the pore throat radii of the samples, MICP analyses were performed on the 12 core plug samples with an AutoPore IV 9500 device at the Unconventional Research Institute of CNOOC EnerTech-Drilling & Production Co. Mercury is a nonwetting liquid, so the mercury volume is equal to the pore volume connect by each characteristic pore radius …”

“…Due to the similar strata, sedimentary characteristics, and structural evolution, the Yingshan Formation and the Yijianfang Formation exposed at the NYG outcrop have been considered to be analogues of the subsurface reservoirs in the Tahe area. 13 The Lower Ordovician Yingshan Formation is divided into two parts by the unconformity (T 7 6 ). The lower part is composed of medium-thin bedded dolomite, algal limestone, and grainstone, and the upper part is made up of medium-thin bedded algal limestone and micritic limestone interbedded with medium bedded fine crystalline algal dolomite.…”

“…In order to measure the distribution of the pore throat radii of the samples, MICP analyses were performed on the 12 core plug samples with an AutoPore IV 9500 device at the Unconventional Research Institute of CNOOC EnerTech-Drilling & Production Co. Mercury is a nonwetting liquid, so the mercury volume is equal to the pore volume connect by each characteristic pore radius. 13 It is accepted that the NMR relaxationc time distribution represents the pore size distribution of the rock sample when the pore space is fully water saturated. 15 Coates et al (1999) provide a detailed explanation of the theoretical basis for the use of NMR.…”

Fractal characteristics of pore networks and sealing capacity of Ordovician carbonate cap rocks: A case study based on outcrop analogues from the Tarim Basin, China

“…This association presents reef and shoal complexes along the carbonate platform margin (Wang et al, 2019). This variation of the depositional environment was probably caused by relative sea-level fluctuation (Chen et al, 1999;Gao et al, 2016).…”