Using Wireline Logging and Thin Sections to Identify Igneous Contact Metamorphism and Hydrothermal Influence on Presalt Limestone Reservoirs in Libra Block, Santos Basin

Abstract

A methodology to confirm the direct contact metamorphism of intrusive rocks and related hydrothermal modification on presalt limestone reservoirs, in Libra Block of Santos Basin, is presented. The approach is based on systemic study of cores, sidewall cores, traditional wireline logging, ECS and NMR of boreholes. Type and characteristics of metamorphism in contact aureoles between intrusive bodies and limestones are confirmed. NMR was used to evaluate porosity change of limestone. Thin sections and ECS were used to identify the precipitant types and to confirm the modification of hydrothermal fluids influences on surrounding limestones. Intrusive rocks are observed in several wells of presalt layers in Libra Block and show contact metamorphic aureoles in the surrounding limestones, characterized by a significant change on the limestone mineral size, rock color, and reservoir pore after marbleization. By combining conventional logging and NMR logging, the thicknesses of contact metamorphic aureoles are determined in multiple wells. They conform to a binomial regression relation with the thicknesses intrusive bodies. The NMR data reveals that igneous contact metamorphism exerts negative influence on pores development for carbonate reservoirs. Reservoir physical properties decrease sharply after thermal baking, generally by 4 to 8 times. The study also confirms that related hydrothermal fluids pervasively develop and have multi-types of influences on limestones, mainly due to silicification, dolomitization and calcification. They mainly brought minerals precipitation in fractures and pores that made limestone reservoir tight. In Libra presalt layers, hydrothermal modification related to intrusive bodies mainly developed at the middle-upper intervals of BVE Fm. Combined with wells correlation sections and seismic data, a state-of-the-art model of igneous contact metamorphism and hydrothermal modification on limestone reservoirs has been developed in this paper. It can help to quantitatively predict the thickness of contact metamorphism aureole around intrusive rocks. The study also clarifies the understanding of the type of hydrothermal effects on the reservoir porosity and how to map the spatial distribution of hydrothermal deposition in the reservoir.