Sedimentary Shattering as Hydrocarbon Migration Avenues in Salt-floored Basins: A New Migration Paradigm
Authors: Lowrie A. , MacKenzie M.G. , Guderion E. , Talbert F.E. , Lerche I. , Pratsch J.C. 
Published: October 2008

The salt-floored basin concept as applied to the Louisiana offshore, with its total dependence on laterally migrating salt, provides a dynamic view of geologic evolution. Salt domes, ridges, and massifs move basinward at rates of one to ten cm/yr, causing extensive fracturing, thermalanomalies, and under- and over-pressured horizons. The everenlarging sedimentary wedge contains its own tectonics; principally basin-spanning shelf-edge growth faults that sole out in shale layers. Advancing salt tongues can createcounter-regional and local growth faults that terminate in the semi-plastic salt. Additional stresses, some by basement faulting, are also active within the salt-floored passive margin.

A geologically reasonable conclusion, derived from the abundance of tectonics, is that the sediments may be thought of as shattered. Porous flowage would be reminiscentof that through a colander, not a sieve. Given the possibility of such massive shattering, it is obligatory to prepare a different paradigm relative to hydrocarbon migration. The now decades old notion that most, if not all, hydrocarbonsmigrate along regional faults must be modified. The shattered sedimentary wedge becomes a conduit for hydrocarbon migration with some concentration of hydrocarbon flow along regional faults, however intermittent that may be. The sedimentary shattering may be so extensive that a new paradigm need be applied to hydrocarbon migration.

Introduction

A classic understanding of hydrocarbon migration is that petroleum matured at a relatively deep level. Oil and gas (both lighter than water) would buoyantly rise through the sedimentary section until reaching a coarse-grained reservoir. When an intervening fault has interrupted this buoyant rise, the hydrocarbons would then migrate up the fault until anothershallower reservoir was found to accept the rising petroleum.

A petroleum-generating system operating (Magoon and Dow, 1994) within the sedimentary wedges of the world implies an amorphous quality to the generation, migration, and, finally, accumulation of hydrocarbons. Active andpassive seeps (Abrams, 1996) suggest that hydrocarbons migrated intermittently and vertically, finding release into the overlying air or ocean (Hovland and Judd, 1988). The widespread (i.e., different basins and extensive areas within each basin (Hovland and Judd, 1988)) occurrences of pockmarks on the seafloor (e.g., Haynes and Baas, in press) are elegant testimony to vertical or near vertical petroleum rising to the sea floor. This paper suggests a new paradigm relative to hydrocarbon migration, which includes migration avenues such as the large number of faults, fractures, and joints generated largely by salt and basement movements and described briefly herein. It is suggested that hydrocarbonsmigrate through shattered sediments as through a colander, not necessarily a sieve.

Pressure Structure

The pressure structure within many petroleum basins suggests an upper, shallow, hydrostatic unit, and a deeper, larger, lithostatic unit (Kuo, 1997, p.224, Powley, 1987; Hunt, 1990) (Fig. 1). A seal or a pressure transition zone (PTZ) separates the two units. Hydrostatic pressure is around 0.47psi/ft (l0.6M Pa/km) and lithostatic pressure begins at approximately 1.06 psi/ft (24.0M Pa/km). Characteristics of the PTZ show a pressure gradient increase from that of hydrostatic units to that of near-lithostatic.