Outcrop-Based Quantification of Short-Scale Heterogeneity in Hybrid Event Beds (HEBs) to Inform Modelling of HEBs-Prone Turbidite Reservoirs

Abstract

Hybrid event beds (HEBs) are a relatively broad family of deposits interpreted as the product of subaqueous sediment gravity flows transitional between turbidity currents and debris flows. Typically HEBs exhibit a tripartite structure, comprising lower and an upper turbidite ‘clean’ sandstones (H1 and H4 intervals) sandwiching a middle debritic interval (H3) in which mud clasts, heterolitic rafts and clean sand injections occur within in a variously argillaceous matrix. Studying HEBs is thus important not only because they represent an important component in many turbidite systems but also for their potential impact on reservoir quality.

This study aims to quantitatively characterise the intra-bed heterogeneity of the full range of HEB types documented in the literature by detailing with cm-scale resolution multiple well-exposed outcrops from different turbidite systems of Northern Apennine of Italy (i.e., the Ventimiglia Flysch Fm., the Mt. Gottero sandstone Fm., Marnoso-Arenacea Fm. and the Castagnola Fm.). Field data collection included: i) logging of 1-m spaced sections detailing thicknesses and sedimentologic characteristics of HEBs facies; ii) drawing of panels (tied to logged sections) of the whole outcrop illustrating in a pictorial way the topology of facies contacts, and iii) acquisition of high-resolution micro-photographs (50x magnification) enabling a consistent estimate of the amount of dispersed clay.

Data from logs and panels of 30 HEBs were then compiled into a database by extracting information on facies and facies boundaries to a 1x1 cm grid. This provided a database that can be interrogated via statistical approaches to pinpoint the features of HEBs most likely to impact reservoir quality, including proportion of component facies, geometry of intra-bed facies boundaries and density and topology of mud clast and clean sand injections in the H3.

The outcomes of this study are useful both for process sedimentology and for applied purposes as they might provide insights into: i) prediction of HEBs lateral variability at a range of scales; ii) probability-aided assessment of HEBs type encountered in boreholes based on thickness and character of component facies, and iii) permeability and connectivity structure of HEB-prone reservoirs

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018