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Petrological and Geophysical Characterization of Tight Volcanic and Pyroclastic Reservoirs: A Case Study From Malang-Tiaohu Sag, Santanghu Basin

Abstract

Tight oil reservoirs have recently been discovered in the Lucaogou Formation of Malang-Tiaohu Sag, Santanghu Basin. Unlike the most tight oil reservoirs that were found in shale, sandstone or carbonate rocks, the lithologies for the tight reservoirs in Santanghu basin are dominantly volcanic and pyroclastic. Petrological and geophysical signatures, which are crucial for evaluating the quality of tight reservoirs, have rarely been distinguished due to the limited amount of tight volcanic reservoirs discovered. This study provides a detailed description of characteristics of tight volcanic and pyroclastic reservoirs by integrating petrological and geophysical methods with additional geochemical and core data. As the results shown, the tight reservoir of Lucaogou Formation shows relative high porosity (16%-20%) and low permeability (< 0.5×10–3 μm2). This contrasting porosity-permeability combination is the result of development of nano-scale pores and throats which make up 88.3% of the whole porosity. Over 62.7% of the samples show oil saturation that is larger than 60% with crude oil density and viscosity around 0.89–0.91 g/cm3 and 56.7–105.0 mPa×s, respectively. X-ray diffraction analysis showed a high percent of quartz and plagioclase components (30–70%) of the reservoir. Together with log data, the brittleness index of reservoir rocks was calculated to be 31%-58%. This is also confirmed by the fact that fractures were developed and became primary migration conduits of hydrocarbon after fracturing. Log responses of the volcanic and pyroclastic reservoirs are characterized by high gamma-ray, sonic and resistivity readings (>40 API, >200 μs/m and >15 Ωm, respectively). Moreover, the reservoir shows strong anisotropy that can be identified from large P-S wave contrast. The criteria developed in this study for identifying and evaluating tight volcanic and pyroclastic reservoirs was actively tested and a favorable source-reservoir pair was then discovered in Malang Sag with the sweet spot located in Niujuan Lake and Mazhong Area. The sweet spot in the member 2 of Tiaohu Formation of Malang-Tiaohu Sag is about 203 km2 with original in-place oil resource estimated to be 2.4×108 t.