Diagenetic patterns in eolian/coastal sabkha reservoirs of the Solimões Basin, Northern Brazil

Abstract

The Juruá Formation (Carboniferous, Solimões Basin) is the largest gas reservoir in Brazil. A petrologic study of the Juruá sandstones from the Urucu area revealed the diagenetic evolution and controls on the quality of the main reservoirs, as well as the relationships among diagenetic patterns, depositional facies and stratigraphic unit boundaries. The sandstones, mudrocks, evaporites and dolostones were deposited within a coastal sabkha environment with pervasive eolian reworking under increasing marine influence and a hot and dry climate. Eolian dune and sandsheet sandstones are the best reservoirs. Four stacked drying/wetting upward cycles were identified, with sabkha facies at the base overlain by eolian sandsheet and dune deposits, which in turn are overlain again by sabkha deposits, commonly eroded by the next cycle. The sandstones are mostly subarkoses and arkoses. Blocky dolomite is the most abundant diagenetic constituent in the non-eolian sandstones. Its distribution shows no relationship with depositional cycle limits. Microcrystalline pore-filling and pore-lining dolomite is more abundant in the eolian sandstones. Stable O and C dolomite isotope values presented by these dolomite cements indicate initial precipitation from a dominantly marine fluid at low temperatures ( δ 18O VPDB values mostly between −1‰ and −5‰; δ 13C VPDB values from −3‰ to +2‰), with increasing contribution of organic matter decarboxylation at higher temperatures during burial ( δ 18O VPDB values between −12‰ and −6‰; δ 13C VPDB from −3.5‰ to −8.9‰). Chemical compaction through intergranular and stylolitic pressure dissolution and cementation by quartz overgrowths are more intense in the non-eolian sandstones. Poikilotopic, patchy, post-compactional anhydrite cementation is more abundant in the eolian sandstones, but pervasive only close to the contacts with interbedded sabkha evaporites. Abundant anhydrite and quartz cementation along the base of depositional cycles contributed to the strong compartmentalization among the reservoir sub-units, but not to introduce internal heterogeneity to the reservoirs. Conversely, dolomite cementation substantially affected the internal porosity and permeability heterogeneity by generating laterally extensive barriers to fluid flow.