Daniel L. Orange

Measurements and modeling of the shallow pore pressure regime at the Sigsbee Escarpment Successful prediction of overpressure and ground-truthing with borehold measurements

Editors note: The material in this paper was prepared and presented (OTC Paper 15201) at the 2003 Offshore Technology Conference, 5–8 May in Houston, Texas, U.S., and is published with permission. Shallow overpressures and shallow water flow (SWF) are significant hazards to deepwater drilling and facilities. In this paper we demonstrate how a combination of geomorphology, hydrogeology, and seismic interpretation has been successfully used in infer shallow overpressures, to develop a testable hypothesis of pore pressure distribution, and to delineate where to collect in-situ data. In-situ pore pressure measurements provide ground-truth data that can validate the model and be used to evaluate the hazards that internally driven failure might pose for facilities development. Several slope failures on the Sigsbee Escarpment in the Mad Dog and Atlantis field areas show a geomorphology distinct from “top-down” slope failures. Specifically, these slumps have steep, amphitheater-shaped headscarps, shallowly sloping bases, sharp inflection points with the surrounding seafloor at both the top and bottom of the headscarp, a linear trend in map view, and linear side walls. These geomorphic characteristics suggest that the slumps are formed by internally driven failure, and that the slumps grow retrogressively by headward migration. Within the Mad Dog and Atlantis field areas, Mad Dog Slump 8 and Atlantis Slump A are the best examples of this morphology; in this paper we will focus our attention on Mad Dog Slump 8 due to its proximity to planned facilities. The types of slope failures discussed in this paper are found on many margins worldwide. Their characteristic morphology indicates that the primary mechanism of slope failure is due to internal forcing (overpressure, weakening), rather than by downslope erosional flow. Internally driven slope failures can provide clues about the shallow overpressure regime, and provide exploration targets. Because the ocean is at constant head, and …

Geologic and shallow salt tectonic setting of the Mad Dog and Atlantis fields: Relationship between salt, faults, and seafloor geomorphology

Many of todays active hydrocarbon provinces are in areas with mobile substrate (either salt or mud). Interpreting the style and activity of the substrate, and its impact on the seafloor and near-surface environment, is central to understanding the geohazards of such field areas.

Interstitial Light Hydrocarbon Gases in Jumbo Piston Cores Offshore Indonesia: Thermogenic or Biogenic?

Abstract Interstitial light hydrocarbon gases were measured to 12 m below seabed in two Jumbo piston cores acquired in deepwater sediments offshore Indonesia. Determinations were made for dissolved methane, ethene, ethane, propene, propane, iso-butane, n-butane, iso-pentane, and n-pentane, as well as for carbon dioxide. Stable carbon isotopic ratios of methane and ethane were also measured. Such gas measurements are typically performed in deepwaters around the world as elements of surface geochemical exploration programs for the purpose of distinguishing biogenic from thermogenic sources in seep gases. Various gas source models have been developed in the literature to aid in accurate interpretation for systems that have been biodegraded, fractionated, and/or mixed during migration. In these models, assumptions are made about the ranges of composition of the end-member gas types and maturities. The compositional ranges of the thermogenic and biogenic end members have been empirically derived from a large database of drilled-hole gas data and various theoretical considerations. In addition to this literature base, tens of thousands of near-surface marine sediment cores have been acquired worldwide and analyzed for interstitial gases on a proprietary SGE basis over the last 35-40 years. Interpretation of these results has helped to fine-tune the accepted end-member compositions of the source models. However, essentially all of the SGE sediment cores have reached a maximum 5 m below the seabed. Our acquisition and analysis of cores down to 12 m has revealed information about gas compositions not apparent from the former SGE coring efforts. In particular, ethane concentrations are in the accepted thermogenic range, but their stable carbon isotopic compositions are not. Near-surface gases formerly interpreted to have an unambiguous thermogenic component are here shown to be purely biogenic. Our conclusions may change the boundaries of the biogenic end member used for the indication of thermogenic gas traces in near-surface sediments.

Applications of multibeam mapping to exploration and production; detecting seeps, mapping geohazards, and managing data overload with GIS

“Multibeam” refers to seafloor mapping systems that use transducer arrays to transmit and receive acoustic information, with individual “beams” insonifying a small portion of the seafloor and the combination of beams providing a swath of seafloor data. In this article we discuss new applications of multibeam technology that use simultaneously collected high‐resolution bathymetric and backscatter data. In particular, such systems can be used to identify seafloor sampling targets, identify mud volcanoes and fault zones, and image modern depositional and erosional systems.