Influence of Diagenesis On the Quality of Lower Cretaceous Pre-salt Lacustrine Carbonate Reservoirs from Northern Campos Basin, Offshore Brazil

Abstract

The genesis and evolution of lacustrine pre-salt carbonate reservoirs, which contain giant hydrocarbon accumulations along the South Atlantic margins, has attracted major research interest. The huge extension and volume, and unusual textural and compositional features, are key elements for understanding the tectonic, structural, stratigraphic, and sedimentological generation and early evolution of the region, as well as potentially of other lacustrine carbonate systems. A systematic petrographic and petrophysical study has been performed on the Lower Cretaceous lacustrine carbonate reservoirs from northern Campos Basin, Offshore Brazil, in order to unravel the main controls on the origin and evolution of pore systems. The main lithologic types recognized in the rift section are bivalve-gastropod grainstones and rudstones, arenites constituted by ooids of syngenetic magnesian clay minerals (mostly stevensite), and dolostones, while stevensitic claystones with calcite spherulites, fascicular calcite crusts, intraclastic rudstones and grainstones, and dolostones are the main lithologic types in the sag section. The eogenetic evolution of bioclastic reservoirs was controlled by the balance between dissolution and neomorphism of the aragonitic bivalve and gastropod bioclasts, favoring either the generation of poorly connected moldic porosity or the preservation of well-connected interparticle porosity. The stevensitic arenites were strongly affected by meteoric dissolution and replacement by dolomite and silica, related to regional uplift and erosion after the rift phase, which generated highly heterogeneous pore systems with moldic, intercrystalline, vugular, and microcrystalline pores. Stevensitic claystones that are replaced by calcite spherulites and dolomite normally show low porosity, but locally constitute reservoirs, where secondary porosity was generated by stevensite dissolution. The precipitation of crystal shrubs of fascicular-optic calcite in coalescent crusts generated growth-framework primary porosity, which was reduced mostly by dolomite cementation, or enlarged by dissolution, enhancing their permeability. Non-coalescent calcite crusts contain abundant syngenetic magnesian clay minerals. Their porosity is related to dissolution of these clays, which generated poor permeability. Intraclastic grainstones and rudstones are compacted and cemented, or rich in clay matrix (''hybrid packstones''). Where they display preserved interparticle primary porosity or matrix dissolution, they may have good porosity and permeability. The heterogeneous dolomitization of both the rift and the sag deposits either destroyed their primary or early diagenetic porosity, or generated high porosity and permeability values in the dolostones. Relationships between replacement and compaction indicate that most of the diagenetic processes occurred during eodiagenesis, controlled mostly by the instability of the aragonite in the bioclastic reservoirs and of the stevensite in the claystones, ooidal arenites, and fascicular calcite c...

First record of carbonates with spherulites and cone-in-cone structures from the Precambrian of Arctic Norway, and their palaeoenvironmental significance

“…212-213). Because of later compaction and tectono-thermal overprint, we can only speculate whether they formed under similar conditions as those described by Wright and Barnett (2015) and Herlinger et al (2017) for the carbonate spherulites from the Lower Cretaceous lacustrine carbonate reservoirs.…”

An Overview of Authigenic Magnesian Clays

“…Further interstratification with kerolite is likely. During eodiagenesis changes in the alkalinity of the pore fluids provoked the dissolution of the Mg-clay [126], whether by decomposition of organic matter supplying CO 2 and/or by crystallochemical transformation of stevensite or stevensite-kerolite mixed layer to talc-like phases [125]. Indeed, with increasing temperature during burial the migration of exchangeable interlayer cations to vacant octahedral sites in the Mg-smectite (Hofmann-Klemen effect) produces a loss of expandability and layer charge originating a talc-like phase.…”

Modelling the impact of depositional and diagenetic processes on reservoir properties of the crystal-shrub limestones in the ‘Pre-Salt’ Barra Velha Formation, Santos Basin, Brazil

“…Individual shrubs are composed of acicular crystals recrystallized as fascicular optical calcite aggregates, with sweeping extinction (Souza et al, 2018). These precipitates generate a growth-framework primary pore system with moderate porosity and good permeability, controlled by large pore throats (Herlinger et al, 2017). It is proposed that shrub size, sorting, and packing, in particular the size of individual shrubs, exerts a primary control on pore size, affecting both porosity and permeability (Rezende and Pope, 2015).…”

“…The crystal-shrub limestones are characterized by multiple phases of calcite, dolomite and quartz cements sitting adjacent to and/or replacing the primary authigenic carbonates, as well as multiple phases of dissolution (Figure 2c;Chitale et al, 2015;Saller et al, 2016;Herlinger et al, 2017;Lima and De Ros, 2019;Souza et al, 2018;Farias et al, 2019). Saddle-dolomite, dawsonite, celestine, and barite occur locally, with dolomite being a major source of primary porosity occlusion (Herlinger et al, 2016;Souza et al, 2018;Farias et al, 2019).…”

“…Saddle-dolomite, dawsonite, celestine, and barite occur locally, with dolomite being a major source of primary porosity occlusion (Herlinger et al, 2016;Souza et al, 2018;Farias et al, 2019). While these diagenetic processes are known to be important determinants of reservoir properties, creating highly heterogeneous changes in porosity and permeability over several orders of magnitude 6 (Herlinger et al, 2017;Terra et al, 2010), the impact of each control has not been isolated or quantified.…”