--> Depositional and Reservoir Character of Mixed Heterozoan-Large Benthic Foraminifera-Siliciclastic Sequences, Middle Miocene, Dominican Republic

AAPG ACE 2018

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Depositional and Reservoir Character of Mixed Heterozoan-Large Benthic Foraminifera-Siliciclastic Sequences, Middle Miocene, Dominican Republic

Abstract

Five middle Miocene 4th- to 3rd-order mixed heterozoan-large benthic foraminifera (LBF)-siliciclastic sequences (DS) developed in response to relative sea-level fluctuations interacting with variable substrate paleotopography. Each sequence is capped by evidence of subaerial exposure (SB), including local paleosols, that evidence relative sea-level falls. The sequences (0–11 m thick) are dominated by shallow marine (1–40 m water depth) carbonate packstone-grainstone facies consisting of LBF, molluscs, echinoderms and bryozoans. This composition reflects the influence of well-documented upwelling in the Caribbean region during this time, which prevented development of photozoan reefs typically formed in tropics. The basal portions of DS1 and DS3 consist of volcaniclastic-carbonate conglomerates and breccias near flanks of volcanic substrate highs that were deposited as debris and grain flows during transgressions. DS2 consist entirely of LBF-bivalve packstone-grainstone. These deposits transition to massive and burrowed LBF packstone-grainstones in lows, away from substrate highs and are in turn overlain by massive and burrowed LBF-bivalve packstones and grainstones. These facies become cross-bedded and contain in-place Kuphus sp. bivalves evidencing shallowing upward to the SB that caps each sequence. DS4 and DS5 consist entirely of massive and burrowed LBF-bivalve packstones and grainstones that contain in-place Kuphus sp. bivalves in the upper portions (shallowing upward) throughout their lateral extent. The absence of gravity flows, the horizontal to gently dipping bedding, and lateral facies consistency suggest most of the original paleotopography had been filled by the time of DS4 and DS5 deposition. The facies characteristics and variable substrate paleotopographic setting in this study are similar to Cenozoic systems forming significant reservoirs in the Caribbean and Indo-Pacific region. Results from this study indicate that sea-level interacting with variable volcanic substrate paleotopography were major controls on sequence stratigraphic architecture and facies distribution. Erosion associated with subaerial exposure resulted in significant lateral thickness variability of sequences. Ongoing petrographic study and analysis of porosity and permeability data from samples in the study area is focused on quantifying reservoir character of facies, determining controls on reservoir character, and developing a static reservoir model for the system.