Gas Anomaly on Seismic: Seismic Interpretation Utilizing Geologic-Seismic Models Resulted in an Onshore In Field"Discovery"

Abstract

In an onshore south Louisiana field, a significant gas well was drilled in an area originally mapped as a [open quotes]low,[close quotes] by reinterpreting the seismic data with the help of depositional concepts and a seismic-geologic model. The gas-bearing sand is interpreted to be of reworked deltaic sand origin. Such sands are produced by the reworking of older abandoned deltaic complexes. The overall gas-bearing sand depostional sequence is comprised of, from base to top, (1) an organic-rich [open quotes]prodelta shale[close quotes] (velocity 7200 ft/sec), overlain by (2) a [open quotes]distributary mouth bar sandstone[close quotes], 350 ft thick in its middle parts, poorly sorted sandstones with abundant finer grained matrix (velocity 10,500 ft per sec.), a characteristic lensoid shape with a concave upward base and a flat top, overlain by (3) the [open quotes]reworked deltaic sandstones,[close quotes] clean, well sorted, quartose sandstones with excellent porosity and permeability, occasionally calci, 40 to 60 ft thick, elongated along depositional strike (velocity 11,500 ft/sec), overlain by (4) [open quotes]transgressive marine shales.[close quotes] The transgressive marine shales are deposited during the destructive phase of the delta system, contain abundant marine organisms, are very calcic, and have higher velocities (compared to prodelta shales) ranging between 7500more » to 8000 ft/sec. The velocities increase going upwards in the section, a reflection of deposition in increasingly deeper marine waters. The seismic criteria that formed the basis for the original [open quotes]synclinal[close quotes] interpretation of the gas zone, once depositionally explained, became the key criteria for the recognition and mapping of the gas anomaly on seismic data.« less