Overview of Carbonate Play Types - Important Characteristics Relevant to Exploration

Paul M. Harris

Carbonate play types, i.e. a family of geologically similar prospects or producing pools that share a common reservoir genesis, are highly varied and therefore offer many exploration possibilities. Common depositional plays and their important characteristics are:

-- Reefs, banks, and buildups (localized biogenic accumulations) occur in elongate trends along windward shelf margins or the edges of intrashelf basins, in isolated equidimensional forms as a fairway on an outer shelf or ramp, and on basement or salt-cored highs within the basin. Requirements of the biota (nutrients, minor turbidity, normal salinity) may further limit distribution. Variations in biota have occurred through geologic time, and a vertical sequence usually reflects growth stages.

-- Carbonate sands (nonskeletal and/or skeletal sand deposits) occur as tidal bars, shoal complexes, or sheets, forming a high energy trend on ramps and shelves, either as a sand-dominated shelf margin or at the edge of intrashelf basins. Such sands may also fringe shallow biogenic buildups and cap deeper-water buildups. Upward-shoaling sequences are common.

-- Downslope debris (shelf, shelf-margin, and slope debris redeposited downslope) forms elongate wedges, lobe-shaped fans, slump blocks, and channel fills along the toe-of-slope and basin margin adjacent to a regional shelf margin or locally surrounding pinnacle buildups. The play is absent in ramp settings unless the ramp is distally steepened. Debris flow and turbidite sequences occur, and porosity is inherited from the shelf and also formed in the subsurface.

Some diagenetic plays and their characteristics are:

-- Dolomite in cyclic sequence (dolomitized tidal-flat and inner-shelf facies within cyclic strata) occurs on the inner portions of ramps and shelves and also behind emergent shelf margins. Evaporite-capped cycles are common, generally producing multiple thin porosity zones.

-- Subunconformity play (secondary porosity formed by dissolution below exposure and unconformity surfaces) is both localized and regional in extent. The former consists of moldic porosity capping upward-shoaling, high-frequency cycles, or vugular and cavernous porosity associated with an intrabasinal unconformity and due to exposure by local tectonism or small-scale eustatic fall. The later is cavernous porosity and brecciation formed during major exposure of tectonic or long-term eustatic origin and associated with an interregional unconformity.

-- Burial diagenesis (porosity formed by dolomitization or dissolution in the deep subsurface) commonly follows joint or fracture patterns, and involves fluids of meteoric, connate, or hydrothermal character. Porosity is highly variable.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995