A THIRD-ORDER UNCONFORMITY WITHIN LOWER ORDOVICIAN CARBONATES IN THE TARIM BASIN, NW CHINA: IMPLICATIONS FOR RESERVOIR DEVELOPMENT

Gao Zhiqian^{1 *}, Fan Tailiang¹, Ding Qunan² and Hu Xiaolan³

¹ China University of Geosciences, Beijing 100083, China.

² National Geological Library of China, Beijing 100083, China.

³ Exploration and Development Department, China Huadian Green Energy Company Limited, Beijing 100016, China.

^* Corresponding author, email: gzq@cugb.edu.cn

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.

Keywords: Palaeozoic carbonates, third-order unconformity, karst reservoirs, Tarim Basin, Lower Ordovician, stable isotopes, trace elements, dissolution porosity.

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