ABSTRACT: Model for Reworked Deltaic Sands--Example from South Lake Arthur, Bayou Gentily, and Bayou Lery Fields
Ram S. Saxena
Reworked deltaic sands are beach-barrier sand bodies that are produced by the reworking of abandoned deltaic lobes. The facies association of these sand bodies makes them ideal reservoirs, and although abundant hydrocarbons are trapped in this type of sandstone, their genesis is seldom understood in the petroleum world.
Criteria and models are developed for the recognition of reworked deltaic sand bodies from the subsurface by comparing a Holocene analog from the eastern periphery of the modern Mississippi delta with an ancient example from the Carboniferous of the northern Appalachian plateau. A simulated seismic model and several subsurface examples are presented.
Reworked deltaic sands are always enclosed in a unique depositional package referred to here as "reworked deltaic sand sequence." This sequence comprises four depositional components. From base to top, these are prodelta shale, distributary mouth bar sandstone, reworked deltaic sandstone, and transgressive marine shale. Each of these depositional components possesses varying lithological characteristics, geometries, thicknesses, porosity, permeability, and sorting, and these variations show up characteristically on electrical logs and seismic lines.
The basal prodelta shales are generally very thick (greater than 300 and often 3000 or more feet). These contain in their upper parts extensive land-derived organic detritus and exhibit a sequence of coarsening-upward grain size and indications of deposition in shallowing waters. The prodelta shales are overlain by distributary mouth bar sandstones that also exhibit a coarsening-upward grain size, have better sorting in their upper parts, and possess greater thickness (200 to 400 ft in the modern delta), and relatively small aerial extent (2 to 4 mi by 2 to 6 mi), whereas the overlying reworked deltaic sands are very well sorted and clean, are 20 to 40 ft thick in the modern delta, have smaller width (0.5 to 0.25 mi in the modern), and a wide lateral extent along depositional strike ( to 20 mi). Shales overlying the reworked deltaic sands are very different from the basal prodelta shales. These are generally calcareous, contain abundant oyster fragments, and reflect deposition initially in shallow brackish waters and later in deep open-marine waters.
In a cross section parallel to depositional strike, a unique arrangement of depositional facies is seen in reworked deltaic sand sequences. In the middle part of the delta, the distributary mouth bar sands and the reworked sands show up as one massive sand body whereas, moving laterally away, the two sand bodies get separated with a transgressive marine shale in between, and the vertical interval separating the two sand bodies increases proportionately to the distance from the middle of the delta lobe. This relationship is produced by the original dip and thickness variations of the older distributary mouth bar. The middle and thicker part of the mouth bar stays within the reach of waves and currents for a longer period of time, and this is the part which is mainly reworked and modifi d. The flanks of the mouth bar are thinner and these drown earlier below the reach of waves and currents.
Reworked deltaic sandstones are ubiquitous in the geologic record, and the geometries and aerial extent of the two sand bodies of this sequence are so different that an accurate development and mapping of such reservoirs would almost be impossible without a precise understanding of their origin.
AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990