Multiphase Calcite Cementation and Fluids Evolution of a Deeply Buried Carbonate Reservoir in the Upper Ordovician Lianglitag Formation, Tahe Oilfield, Tarim Basin, NW China

Abstract

Oil and gas have been found in the Upper Ordovician Lianglitag Formation carbonates in the Tahe Oilfield, Tarim Basin, NW China. This study documents the origin of diagenetic fluids by using a combination of petrology, SIMS, fluid inclusion, and radiogenic isotope analysis. Six stages of calcite cements were revealed. C1-C2 formed in marine to early burial environments. C3 has relatively low 18 O VPDB values (−8.45‰ to −6.50‰) and likely has a meteoric origin. Meteoric water probably fluxed into aquifers during the Early Paleozoic and Late Paleozoic uplift. C4 has 18 O VPDB values typically 3‰ higher than those of C3, and probably formed during shallow burial. C5 displays relatively negative 18 O VPDB values (−8.26‰ to −5.12‰), and the moderate-tohigh fluid-inclusion temperatures imply that it precipitated in burial environments. C6 shows homogenization temperatures (up to 200 ∘ C) higher than the maximum burial and much lower salinities (<10.61 wt% NaCl), which may suggest that the fluid was deeply recycled meteoric water. The average 87 Sr/ 86 Sr ratios of fracture-and vug-filling calcite cements are much higher, indicative of incorporation of radiogenic Sr. Caves and fractures constitute the dominant reservoir spaces. A corresponding diagenesis-related reservoir evolution model was established that favors exploration and prediction.

Fracture, Dissolution, and Cementation Events in Ordovician Carbonate Reservoirs, Tarim Basin, NW China

“…Giant volumes of oil and gas are contained in Ordovician carbonate rocks of the Tarim Basin, NW China, especially in the Tabei Uplift, where proven oil/gas reserves surpass 3 billion tonnes of oil-equivalent [1]. Well drilling responses such as bit drops and mud losses indicate that high-porosity and high-permeability zones, including large (>10 m) cavities, are present in deeply buried (>6000 m) reservoirs [2][3][4][5]. Many economic carbonate reservoirs are housed in rocks that preserve early dissolution porosity produced by meteoric diagenesis-karstification-prior to significant burial [6][7][8][9][10][11][12].…”

“…The origin of the deep pore networks in carbonate rocks of the Yijianfang Formation in the central Tarim Basin is debated [4,5,[21][22][23]. Karstic features are present in Middle Ordovician carbonates within the nearby Lunnan and Tahe oilfields [2,[24][25][26][27].…”