Using 3D seismic data to map shallow-marine geohazards: a case study from the Santos Basin, Brazil

Abstract

This paper documents a study that used high quality, conventionally processed, speculative 3D seismic data for two specific purposes: first, to identify and avoid any health, safety, security and environmental (HSSE) issues regarding the water bottom and, secondly, to avoid shallow (<1 s below the mud line) geohazards when drilling the wells. The study integrated existing legacy well data, new 3D data from the BM-S-52 concession and a targeted programme of water-bottom grab samples from the BM-S-52 concession, Santos Basin, offshore Brazil. Two aspects of the analysis are described: the first addresses the problem and solution adopted for characterizing the water-bottom environment in this area; the second addresses the interpretation of the data for shallow geohazards. The results of this case study have been accepted by the regulatory authorities as representing sufficient evidence of risk evaluation that no further dedicated site-survey data acquisition is required. This has led to savings in time and in costs which would otherwise have been expended on deep-water site-survey campaigns. Furthermore, the use of 3D seismic data for well-location planning brings benefits in improved imaging of the near-mud-line geology, full coverage over the whole concession, operational flexibility for changing well location and timely availability of data. The analysis workflow may also be applicable in other, similar deep-water areas.

An extensive pockmark field on the upper Atlantic margin of Southeast Brazil: spatial analysis and its relationship with salt diapirism

“…The existence of pockmarks in the region was initially confirmed by a single-beam echo sounder survey ( Cooke et al, 2007 ), with further examples scattered across slumped areas in the Santos Basin identified by Sharp and Badalini (2013) . A series of craters identified by Bastos et al (2013) on the Abrolhos Bank, Eastern Brazilian Shelf, and termed “buracas”—meaning holes in Portuguese.…”

Geohazard detection using 3D seismic data to enhance offshore scientific drilling site selection

“…The geohazard assessment workflow provides a blueprint that maintains the high level of safety assessment associated with IODP drilling by incorporating modern 3D seismic data manipulation and interpretation techniques to take full advantage of the available data (e.g. Heggland et al, 1996;Sharp and Samuel, 2004;Sharp and Badalini, 2013) (Fig. 5 and Table 1).…”

“…The expansion, especially with regards to new 3D seismic reflection data coverage, provides an opportunity to use these data for reasons beyond their original commercial purpose. This includes regional geological mapping and geohazard assessments allowing site identification and derisking for scientific boreholes (Dutta et al, 2010;Selvage et al, 2012;Hovland et al, 1998).…”

“…A geohazard assessment involves the geospatial quantification of drilling hazards, ideally through the use of densely sampled 2D or 3D reflection seismic data (Aird, 2010;Khan et al, 2018;Selvage et al, 2012;Heggland et al, 1996). In the past, the required resolution to detect small but potentially hazardous features could only be provided through dedicated high-resolution site surveys across potential drill sites using such tools as sub-bottom profilers, small-volume airguns, or sparker reflection systems (Jensen and Cauquil, 2013;Parkinson, 2000).…”

“…The processing of these datasets has improved sufficiently to provide a vertical resolution that approaches that of a traditional site survey and often provides a horizontal resolution exceeding that of even closely spaced 2D seismic site surveys (Games and Self, 2017;Oukili et al, 2019). Three-dimensional seismic surveys thus minimize the need for additional data, except for particularly complicated areas (Hill, 1996;Selvage et al, 2012;Sharp and Badalini, 2013;Williams and Andresen, 1996;Roberts et al, 1996).…”