Summary

One of the significant challenges in the characterization of the Cambrian Longwangmiao Formation, Sichuan basin, South West China was to recognize the lateral and vertical variability of lithofacies and secondary porosity development and then construct the spatial distribution trends of these heterogeneous carbonate gas reservoirs. Conventional openhole logs alone are often not sufficient to differentiate the different lithofacies and secondary porosity in the studied reservoirs and seismic data has limited resolution to provide needed information.

In this paper, logging-while-drilling electrical image logs from one horizontal well and wireline electrical image logs from six vertical wells have been used to characterize heterogeneous carbonate gas reservoirs. The horizontal well cuts through more high-angle fractures and more lateral secondary development units, while the vertical wells drill through vertical variability of lithofacies and secondary development zones. High-angle fractures and vuggy porosity can be well recognized on image logs in multiple wellbores. In addition, the relationship between secondary porosity and lithofacies is better understood by the calibration of core to the electrical image logs. With the help of 3-D modeling, the distribution of the secondary porosity in the field can be predicted. As a result, a much clear picture on the critical secondary porosity trends for the studied complex carbonate reservoirs has been achieved through the integration of electrical image logs, core, conventioanl open hole logs, as well as 3-D modeling technique. This approach is appliable to characterize other heterogeneous carbonate reservoirs.

Introduction

The carbonate gas reservoir of the Cambrian Longwangmiao Formation forms an extra large early Paleozoic natural gas field in the central Sichuan basin of China. This carbonate gas reservoir is another essential natural gas resource in unconventional gas exploration and development besides the marine shale gas reservoir of the early Silurian formation in the central Sichuan basin. The carbonate gas field of the central Sichuan basin covers 7,500 km2, the original gas in place (OGIP) is estimated to exceed 60,000×108 m3. By the 2013, the gas-bearing area has been proved 779.86 km2, the proven OGIP is 4403.83×108 m3, and the recoverable gas is 3,082×108 m3 (Du et al. 2014).

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