--> Diagenesis of Pre-Salt Carbonates, Brazil Margin – Challenging Paradigms of Reservoirs in Evaporite Basins

AAPG ACE 2018

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Diagenesis of Pre-Salt Carbonates, Brazil Margin – Challenging Paradigms of Reservoirs in Evaporite Basins

Abstract

Many of the world largest oil and gas fields are sealed by basin-filling, marine fed evaporites. The study of these reservoirs worldwide has resulted in several paradigms of carbonate diagenesis in evaporite basins, some of which include i) dolomitization of evaporite-sealed reservoirs, ii) calcium sulphate diagenesis, iii) pore plugging by evaporite minerals, and iv) sulphate reduction and H2S generation.

The integration of core, petrographic, reservoir quality data with isotope and fluid inclusion microthermometry data from the pre-salt non-marine carbonates of the Santos Basin margin challenges these long-held concepts.

Sealed by basin-filling, marine-fed evaporites, the Barra Velha (BVE) Formation carbonates comprise thick and laterally extensive non-marine carbonates, with exceptionally good reservoir quality and lateral continuity. The BVE carbonates are predominately limestones, comprised of three main carbonate components, micritic carbonate mud, calcite spherulites, and calcite crystal shrubs, which are combined to form four main lithofacies- 1) Grainstones/Rudstones, 2) Crystal Shrubs, 3) Laminated Spherulites and 4) Calcimudstones.

Carbonates, silica and Mg-silicates are primary components of the sedimentary system. Mg-silicates (e.g. Talc-Stevensite) are one of the principal porosity destructive components, quartz (chert, chalcedony), calcite and (minor) dolomite are the main cements. Notable throughout the succession are i) the ‘survival’ of lots of open pore space with little or no cements, ii) the rarity of stylolites and iii) the absence of primary and diagenetic sulphides and chlorides.

Isotope and fluid-inclusion data reflect “normal” diagenesis overprinted by high-temperature systems (i.e., fault conduits) which have played a local role in Pre-Salt silica emplacement. Reservoir quality is largely a result of primary porosity preservation related to depositional lithofacies, and in the Santos later diagenetic alteration has done little to change this picture.

Since most the porosity is of primary origin, processes that allow porosity preservation despite the burial depth and the record of several diagenetic events, need to be investigated to better predict the distribution of reservoir sweet spots.