Fifteen thin sections were generated from mudstone cuttings of well OCS-G-3733 A-6 (API: 427034015800). Fourteen thin sections were created using cuttings from well OCS-G-6042 API 4270340281l, which is not shown on the map above, but is located near the Matagorda Island wells. However, the petrographic technicians were unable to sufficiently polish the thin sections due to their unconsolidated nature. The poor quality of the thin sections made petrographic and texture studies very difficult.
The high potential value for seal analyses (i.e., in light of the small number of core samples) necessitated a second attempt to generate polished thin sections from mudstone drill cuttings.
Ten cuttings from a well (API 4270600020) close to the first P-cable survey area were selected within the depth range 4900 to 10105 ft. The samples were moderately polished for SEM examination.
The analyses were carried out using a field-emission SEM, an FEI Nova NanoSEM 430, at The University of Texas at Austin. Both Secondary Electron (SE) and Back-Scattered Electron modes were used to show mudstone texture. A system of two, energy-dispersive, X-ray detectors was used in conjunction with SEM to produce elemental maps and consequently, help to identify mineral phases. Back-Scattered Electron images and EDS (energy dispersive spectroscopy) elemental maps were obtained from the thin sections, which yielded useful information on the texture and lithology of the fine-grained portion of the examined samples.
Thin section texture shows large variation. Grain size ranges from clay to fine sand. Silt and sand abundance varies significantly. Figure A shows a claystone with few silt grains while Figure F shows a sample containing over 80% silt grains.
SEM imaging provides a qualitative assessment of fabric alignment. The samples show large variations in degree of fabric orientation. For example, the sample in Figure A shows a high degree of clay alignment in the vertical direction in the image, while the silty claystone in Figure G shows weak clay alignment. Apparently, higher silt/sand content leads to poor alignment. However, silt/sand content is not the only control. For example, Figure A and B are from the same cuttings interval (~ 30 ft), but the section in Figure A is highly aligned and in Figure B the claystone is poorly aligned. Depth control on clay alignment is not obvious either. Figures A and B show quite different degrees of alignment within the same sample. Figure A is from 4900-4930 ft and it shows better alignment than the deeper samples (e.g. Figures C, G, and H).