Carbonates That Are No More: Silicified Pre-Salt Oil Reservoirs in Campos Basin (Brazil)

Abstract

In Campos Basin pervasively silicified, shallow-to-proximal bench slope lacustrine carbonates are found alternating with profundal lacustrine laminated calci/dolosiltstones. Original carbonates were composed by clotted thrombolites, intercalated with minor pack-grainstones to rudstones.

An early silica (limnic silica-1) event is represented by direct lacustrine precipitates as discrete, fine-grained opaline crusts covering topographic paleo-surfaces, thought to occur during alkalinity crises triggered by humid episodes and silicic acid destabilization.

A syn-sedimentary-early diagenetic silicification event (early diagenetic silica-2) post-dates early carbonate corrosion. Silica textures vary from fine-grained, carbonate fabric-preserving to almost-fully replacive fabric destructive silica, with some intensely corroded carbonate remains. Syn-sedimentary silica textures include (1) dominant opal-chalcedony-(mega)quartz cement passive filling sequences, representing phreatic (hydrothermal pond) ambients; (2) scarce, vadose opal-cemented geopetal filling structures in previously corroded thrombolitic/stromatolitic carbonates; and (3) pseudomorphosed bladed/raft calcites by silica, indicative of hydrothermal boiling in a sub-lacustrine sinter ambient. Silica diagenesis is dominated by transformation of opal-A to microcrystalline quartz (≈50.000 years), with concomitant destruction of microtextures, changes in porosity and geochemical alteration.

Late hydrothermal silicification (hydrothermal silica-3), including quartz veins and stockworks, hydrothermal breccias and vugs, overprinted the already silicified horizons. Late silicification is caused by the inflow of high temperature (90-170°C), medium-to-high salinity (10-20wt% NaCl eq.) hydrothermal plume, with coeval hydrocarbon migration (black-oil to condensate). This hydrothermal plume caused: (1) hydraulic fracturing, porosity formation and late mega-quartz cementation; (2) recrystallization of the early diagenetic silica textures; (3) corrosion and silicification of the remaining carbonate components; (4) generation of in-situ pyrobitumen after thermal degradation of liquid hydrocarbons. The age of the hydrothermal plume (adularia Ar-Ar step heating) spans from 107.0±1.0 to 104.3±0.8Ma (mid Albian). The hydrothermal plume also hydraulically fractured the upper tight, thin kerolite-rich profundal lacustrine sediments (healing phase), causing mineral neomorphism (talc, dedolomite) and pervasive silicification.

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