Redefining Geological Evaluation in Deepwater Using New High-Resolution Borehole Imaging Technology
In Gulf of Mexico wells drilled with oil-based mud (OBM), legacy OBM-adapted microresistivity imagers provided a technological step forward for formation evaluation. However, 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 deepwater Gulf of Mexico wells, 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 are observed. 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, cross-beds and other flow-regime indicators are now clearly observed. In 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, demonstrates complex depositional changes occurring in the basin. With legacy technology, these various deposits were nearly impossible to characterize accurately.
The examples discussed in this paper demonstrate the ability of the new OBM borehole images to allow enhanced interpretation of reservoir sand architecture, within the framework of surrounding structural elements, to more accurately position them within the depositional system. As such, they further enhance the detail and understanding of other petrophysical measurements, allowing the asset geoscientist an improved realization of these very complex reservoirs.
AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015