Fundamental stratigraphic, sedimentologic, and petrophysical elements of heterozoan carbonates: Grain-rich fining- and shoaling-upward cyclothems and clinothems.

摘要

This research documents Pliocene and Miocene outcrops from SE Spain to evaluate controls on stratigraphic architecture and petrophysical properties of carbonate rocks of the heterozoan association. It shows that grain-rich, fining updip clinothems and fining-upward cyclothems are expected to form in heterozoan carbonate systems, and depending on degree of diagenetic alteration, some of these fundamental units of stratigraphic architecture have predictable petrophysical properties.;The Pliocene outcrop is a lowstand shaved-shelf system, forming a single prograding clinothem. Ten lithofacies are distributed along six depositional profiles that reflect four different environmental settings. Hypothetical progradation of each of these depositional profiles would form six end-member fining-updip and fining-upward, grain-rich clinothems. Proximal facies are the finest, and are composed of sorted bryozoan-coralline algae packstones. More distal facies are poorly sorted, rhodolith and bivalve rudstones.;Miocene deposits are composed of seven cyclothems, which drape and onlap a gently sloping surface. In proximal settings, cyclothems consist of poorly sorted rudstone facies that coarsen upward and then fine upward to a cross-bedded, well-sorted packstone. In medial to distal settings, cyclothems are coarse rudstone at the base, which fines upward to packstone. The most distal cycles consist of basal hemipelagic wackestones that are overlain by rudstone and packstone facies. Most typically, the cyclothems are coarse-grained facies at the base, interpreted to form during transgression, and overlain by a finer packstone, interpreted to form during a relative fall in sea level. The fine grain size is interpreted to form as waves rework coarse sediments into sorted, abraded sands, creating a fining-upward trend. Bryozoans are most abundant in proximal areas and average 54% of the total sediment, bivalves dominate medial regions with 40%, and coralline algae in distal regions with averages of 33%.;The shoaling- and fining-upward, grain-rich cycles from the Miocene develop primarily from relative sea level and paleotopographic position. Using tilt-corrected paleotopography and interpretations of depositional environment from lithofacies, a quantitative sea-level curve has been developed and indicates typical sea-level amplitudes of 24-67m, with two outlier amplitudes that reach up to 116m.;In the Pliocene clinothem, sorted packstones have 51% average porosity and 6,230md median permeability, whereas rudstone facies have 42% and 2,538md. Dolomitized Miocene packstones have 32% average porosity and 313md median permeability, and dolomitized rudstones have 36% average porosity and 115md median permeability, whereas least-altered Miocene packstones have 43% average porosity and 5,279md median permeability, and rudstones have 40% average porosity and 1,800md median permeability. These data indicate that sorted packstones have the highest original petrophysical values. Where diagenetically altered, packstones are more altered than genetic rudstones and therefore have higher variability. Originally high permeability of packstones appears to allow for enhanced diagenetic fluid flow, leading to increased alteration. In Pliocene and Miocene deposits that have not been extensively altered, and that lack calcisilt-rich caps, vertical depositional profiles show an upward increase in petrophysical values associated with upward shoaling. In diagenetically altered deposits, this predictable trend becomes more variable, and is commonly reversed.;These data indicate that heterozoan systems that have not been extensively diagenetically altered form petrophysical trends that are predictable based on the stratigraphic architecture and facies distribution of grain-rich shoaling- and fining-upward cycles. These outcrop analogs have been developed into eight 3D Petrel(TM) reservoir-analog models and facies models to aid in predicting the distribution of reservoir character in similar deposits in the subsurface. 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.