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Advanced Geological Evaluation in Deep Water Using New High-Resolution Borehole Imaging Technology

Abstract

Technological breakthroughs have been the foundation of advanced achievement within each segment of the oil and gas industry. In deepwater Gulf of Mexico (GoM) wells drilled with oil-based mud (OBM), legacy OBM-adapted microresistivity imagers were a technological step forward for formation evaluation, but the resultant images lacked the high-resolution and borehole coverage necessary for detailed geologic interpretation, leaving much to inference. Now, a newly engineered wireline OBM-adapted microelectrical borehole imager has been deployed that can acquire high-definition, high-coverage images in any well drilled with OBM. The images produced have proven to be a revolutionary advance for borehole image-based geological interpretation, including enhanced structural evaluation and especially detailed sedimentological assessment. This paper details examples from wells in the deepwater Gulf of Mexico, wherein the new OBM-adapted borehole images were acquired as part of wireline logging suites. Because of the high resolution and the large borehole coverage of the images, fine details of the reservoir sands as well as the intervening shales were observed. Within the sands, the differing modes of sedimentation can be clearly distinguished; lower energy sands are easily discriminated from higher-energy channel-axial deposition and irregularly-bedded or chaotic sands. Channel scours, imbricated rip-up clasts, ripples and other flow-regime indicators are now clearly observed. In the shales, where previous imagers have been seriously challenged, the new photorealistic images crisply delineate the laminations and bedding to permit accurate structural dip determination. Additionally, the differentiation of the quiet environment sediments from the high energy deposits – including debris flows and mass transport deposits – brings into light the depositional changes occurring in the basin. With legacy technology, these latter deposits were nearly impossible to characterize. The examples discussed in this paper demonstrate the ability of the new OBM borehole images to provide enhanced interpretation of depositional environments of reservoir sands. This opens a whole new avenue of information to better recognize the nature of the reservoirs in the GoM, and more accurately position them within the depositional system. The asset geoscientist is now able to come much closer to the reality of these very complex reservoirs.