--> ABSTRACT: Seismically Defined Incised Valley Fills in a Mixed Carbonate-Siliciclastic System, The Southern Belize Lagoon: Reservoir Implications, by Dominic Esker, Donald M. McNeill, Gregor P. Eberli; #91020 (1995).

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Seismically Defined Incised Valley Fills in a Mixed Carbonate-Siliciclastic System, The Southern Belize Lagoon: Reservoir Implications

Dominic Esker, Donald M. McNeill, Gregor P. Eberli

High resolution single-channel seismic data reveal incised valley fills ranging in thickness from 20 to 50 meters, all of which contain the same repetitive sequence of seismic boundaries and facies. The late Pleistocene to Holocene-aged valley fills contain from deeper to shallower: 1) an irregular basal unconformity (the sequence boundary of the last lowstand); a) a fluvial facies with, in some places, thick, possibly reservoir quality channel sands and gravels; in some valleys 2) a transgressive surface overlain by b) finer estuarine deposits; 3) an irregular tidally-cut ravinement surface with channels; c) a tidal facies with locally thick, well-sorted reservoir quality sands; 4) a very flat marine-cut ravinement surface; and d) a uniform marine facies of well-sorted sands and modern marine muds near and at the present sea floor. The reservoir quality facies contained in these valley fills are on the order of tens of meters thick and in some instances, such as the tidal sands throughout much of the lagoon, show good heterogeneity and predictability. This facies is rarely eroded by the relatively flat lying marine ravinement surface above. Other reservoir facies, such as the fluvial channel sands, pinch and swell and are much less predictable t roughout much of the lagoon due to the effects of local tectonics on the basal unconformity and re-incision by the overlying tidal ravinement surface.

Statistically, these valley fills reflect local tectonics, drainage pattern, and sediment supply and composition which dramatically affect the resulting distribution of reservoir quality facies. Therefore, the southern lagoon should be viewed as three separate subsystems: the Inner Channel zone along the mainland; the Victoria Channel zone just inside the barrier reef; and the Southern Valley zone just north of the Gulf of Honduras. The valley fills of the Inner Channel zone are relatively thin and contain a very irregular, segmented basal unconformity (1). The facies pattern in this zone reflects a tectonic overprint on this zone as most of the deposition is restricted to fault-created lows and occurs early in the valley history. This has resulted in discontinuous lenses of fluvial c annel sands and relatively thin tidal sands. The Victoria Channel zone valley fills have a flatter, more poorly developed basal unconformity (1) and its fluvial facies (a) differs in character (fewer true channels and channel facies and more widespread drape-like deposits). This zone shows no structure at all and although it lacks much channeling, it does contain locally thick tidal (c) and marine (d) deposits. This may be due to local production and deposition of carbonates by the nearby reefs (barrier and lagoon reefs). The Southern Valley zone contains an often fault-controlled, irregular basal unconformity (1) well-developed channel-fill facies (a).

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