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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 64 (1980)

Issue: 5. (May)

First Page: 780

Last Page: 780

Title: Geologic-Seismic Exploration Model for Reworked Deltaic Sandstones--Excellent Subsurface Reservoir: ABSTRACT

Author(s): R. S. Saxena

Article Type: Meeting abstract

Abstract:

Reworked deltaic sands are produced by the reworking of abandoned deltaic lobes. Criteria and models are developed for the recognition of reworked deltaic sand bodies on subsurface log and seismic data. A geologic-seismic model and a subsurface prospect example is presented.

A reworked deltaic sand sequence is composed of four depositional components. From base to top these are, prodelta shale, distributary mouth bar sandstone, reworked deltaic sandstone, and transgressive marine shale. The basal prodelta shales possess lower velocities and commonly lower density and resistivity, and are generally very thick--greater than 300 ft (91 m) and commonly 3,000 ft (914 m) or more. The thickness of overlying distributary mouth bar sands generally ranges between 200 and 400 ft (61 and 122 m) and they are depopod-shaped--2 to 4 mi (3.2 to 6.4 km) along depositional dip and 2 to 6 mi (3.2 to 9.6 km) along depositional strike. Mouth bar sands display moderate to poor sorting in contrast to the overlying reworked deltaic sands which are well sorted, possess excellent orosity and permeability, and make better reservoirs. Reworked sands are elongated along depositional strike--5 to 15 mi (8 to 24 km) long and 0.25 to 0.5 mi (0.4 to 0.8 km) wide. Their thickness ranges between 20 and 60 ft (6 and 18 m). Marine shales overlying the reworked deltaic sands possess higher velocities and exhibit higher resistivity on electric logs. They are calcareous, contain abundant oyster fragments, and reflect deposition initially in shallow brackish waters and later in deep open-marine waters.

The contrasting lithologies, velocity differences, and the geometries of various depositional units of reworked detrital sand sequences provide excellent clues for their recognition on log and seismic data.

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Copyright 1997 American Association of Petroleum Geologists