New Approaching for Rock Types and Lithofacies Synergy for the Hollin Formation, Palo Azul Field, Ecuador

Abstract

AbstractThis paper discusses a different approach to defining rock types and lithofacies performed for the Hollin formation located in the Palo Azul field of Ecuador. The study includes a spectrum of depositional environments, which significantly influences geometry, diagenesis, and quality of the Hollin reservoir. This work integrates lithofacies and petrophysical properties using well log data, core, and sedimentological analyses to define rock types.Sedimentological analyses were performed to describe existing cores, thin sections, and X-ray diffraction results. Flow units were identified using flow zone indicator (FZI) and reservoir quality index (RQI) methods. These units were classified as functions of flow capacity (K/Phi) based on capillary pressure (Pc) and other special lab analyses. Capillary pressure enabled the grouping of rock flow units in accordance with the K/Phi ratio, similar RQI, and irreducible water saturation (Swirr) values. These rock types (RT) were correlated with the lithofacies identified using sedimentological analysis.The transgressive Hollin formation sequence includes fluvial mid-grained sandstones with cross stratification, tidal mid- to fine-grained sandstones with mud drapes and organic matter, and marine glauconitic sandstones with/without calcareous cement. Integrated sedimentological and petrophysical analysis defined five rock types (RT1-RT5). RT1 and RT2 correspond to tidal quarztarenites and fluvial sandstones with few discontinuous clay laminae, high permeability, and high porosity values. RT3 includes tidal fine-grained sandstones with abundant clay and organic matter in the matrix, which decreases flow capacity. RT4 is characterized by very fine- to fine-cemented tidal sandstones in which the diagenetic events close the porosity and permeability. RT5 is a muddy sequence of tidal/fluvial environments, marine shales, and well-cemented glauconitic sandstones. These rocks present negligible permeability and low porosity. In conclusion, the reservoirs deposited in tidal bars and fluvial channels have major flow capacity and storage characteristics, whereas the rocks of the shallow marine and sand flat environments present poor reservoir quality.This integrated petrophysical and sedimentological work presents an alternative method for identifying rock types by using flow capacity and the integration of conventional core data, sedimentological analysis, petrographic and diagenetic description, capillary pressure, well logs, and reservoir information. The results from this method were incorporated in the geocellular model for reservoir simulation.