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.
The oldest petroleum discoveries on the Brazilian offshore margin are in Paleogene reservoirs of the Guaricema and Dourado fields in the structural Mosqueiro Low, the Southern Sergipe-Alagoas Basin. Study of well logs and biostratigraphic data of the Eocene Series allowed its subdivision into three stratigraphic intervals representing the Ypresian, Lutetian-Bartonian, and Priabonian stages. The Ypresian depocenter is in the Dourado Trough, whereas the Lutetian-Bartonian and Priabonian depocenters are in the Vaza-Barris Trough, controlled by halokinesis in the first one and basement deformation in the others. The Ypresian interval mainly comprises retrogradational deposits related to the Early Eocene Climate Optimum. The Middle-Eocene Basal Unconformity is the main erosive feature in the Series. The Lutetian-Bartonian and Priabonian depositional systems are progradational. Their deposition is related to basement uplift due to both intense deep-water magmatism and to the Incaica Phase of the Andean Orogeny.
Libra carbonate reservoirs, besides its great heterogeneity, are also characterized by occurrence of igneous rocks, as a challenge to reservoir modeling and production performance prediction. The objectives of this paper are three fields:1) To better understand the genetic cause of magma events and its relationship with fault activity;2) To minimize the uncertainties of the outcomes from geophysical and petrophysical methods;3) To enhance the reliability and accuracy of igneous rocks’ prediction.
Several semi-quantitative to quantitative assessment methods have been attempted and employed to evaluate the fault activity. The result from fault growth index reveal that the Class-I faults are continuously active from PIC (PiÇarras Fm.) to BVE (Barra Velha Fm.), throughout the whole rift period, but the time when they have the highest activity intensity value is getting later from west to east. In NW structure of Libra, their most intensity appeared during the period of PIC deposition. In Central structure, they show their highest activity values mainly during ITP (Itapema Fm.) stage. And the Class-I faults in SE structure continue to be active even during the deposition time of BVE. The eight class-II faults show their movement mainly during the PIC and ITP period and they were no longer active during BVE stage. The two Class-II faults in NW structure were more active, with a largest value of activity intensity during PIC while the same order faults in Central and SE structure lasted for even longer time, and their highest intensity occurred in ITP deposition period. All the faults, including Class-I faults and Class-II faults, usually have a longer activity duration and a higher intensity in their middle part, and a relatively shorter activity time and a lower intensity value in their two endpoints (Figure.3).
An igneous rocks genetic geological model is built up. The Aptian volcano was interpreted as a type of fissure event when the magma was distributed along the regional faults. The Class-II and some Class-I faults and their movement contribute to the formation of Aptian craters and then controlled the distribution of afterward extrusive rock. The Class-I faults and their later reactivation play a key role in the distribution of Santonian intrusive igneous rocks. Under the guidance of such model, methods of multi-scale and multi-disciplinary could be used to predict igneous rocks, which could significantly and effectively reduce the uncertainties of seismic data and enhance the reliability and accuracy of igneous rocks’ prediction.
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