Fault Activity and its Influences on Distribution of Igneous Rocks in Libra Block, Santos Basin: Semi-Quantitative to Quantitative Assessment of Fault Activity Based on High-Resolution 3D Seismic Data

Abstract

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 The eight class-II faults show their movement mainly during the PIC and ITP period and they were no longer active during BVE stage.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.