Fundamental Controls on Modeling Reservoir Properties of Fining-Updip Heterozoan Carbonates

Abstract

Outcrops of two Neogene systems in SE Spain provide analogs for modeling reservoir properties of heterozoan carbonates. A Pliocene system is dominated by clinothems, which have six depositional profiles comprising a fining-updip facies trend. Proximal facies are sorted bryozoan-bivalve packstones, which change downdip into poorly sorted, rhodolith and bivalve rudstones. A Miocene system is composed of stacked cycles; each cycle consists of basal transgressive coarse gravels and an upper regressive crossed-bedded packstone. As shoaling occurs, waves rework coarse sediments into sorted, abraded sands, creating a fining-updip trend. Miocene cycles form patterns that are predictable, but dependent, on paleotopographic position. Three facies comprise proximal cycles: two poorly sorted rudstone facies that coarsen upward and abruptly change to a cross-bedded, well-sorted packstone. Medial and distal cycles are composed of two facies: a coarse rudstone overlain by a packstone facies. The most distal cycles consist of basal hemipelagic wackestones that are overlain by rudstone and packstone facies. Grain constituents also form predictable trends, as bryozoans are most abundant in proximal areas (ave 38%), bivalves in medial regions (ave 30%), and coralline algae in distal regions (ave 25%). Diagenetic impact is important, as dolomitized and calcite cemented Miocene deposits have mostly been altered from original depositional petrophysical character, whereas Pliocene clinothem deposits are close to original depositional values. Clinothem packstones have petrophysical values of 47% porosity and 5d permeability, whereas rudstone facies have 42% and 4d. Dolomitized Miocene packstones have 32% and 0.9d, and dolomitized rudstones have 36% and 1.1d, whereas least-altered Miocene packstones have 43% and 5d, and rudstones have 40% and 1.8d. These data indicate that sorted packstones have the highest original petrophysical values. The original high permeability appears to allow increased diagenetic fluid flow, leading to increased alteration. Grain sorting, size, and constituents, dolomite content, and bedding are the primary controls on petrophysical values. These data have been utilized to develop sedimentologic models and 8 detailed Petrel models. The results establish controls that predict facies distribution and depositional trends to enable the modeling of petrophysical properties of heterozoan reservoirs in the subsurface of SE Asia, offshore Venezuela and elsewhere.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016