Tectono-Estratigrafia e Modelação de Sistemas Petrolíferos da Bacia do Porto

UNIVERSIDADE DE LISBOA FACULDADE DE CIÊNCIAS DEPARTAMENTO DE GEOLOGIA

TECTONO-ESTRATIGRAFIA E MODELAÇÃO DE SISTEMAS PETROLÍFEROS DA BACIA DO PORTO João Pedro Malveiro Casacão

Dissertação orientada por: Professor Doutor Nuno Lamas de Almeida Pimentel

Mestrado em Geologia (Especialização em Estratigrafia, Sedimentologia e Paleontologia)

2015

Abstract

Abstract This work consists mainly on the reevaluation of petroleum potential of Porto Basin, located on the northwestern sector of the Portuguese occidental margin. This analysis consisted on the identification of the main tectono-stratigraphic events that controlled the basin’s evolution and the adjacent deep offshore sector, as well as maturity analysis of the main recognized source rock intervals. The Porto Basin had some hydrocarbon exploration record at the end of the XX century, until 20 years ago, with five exploration wells drilled and several 2D seismic acquisition campaigns that lasted until 2001. The seismic lines reinterpretation confirmed the existence of a NNW-SSE trend basin on the continental shelf, bordered to the east by the Porto-Tomar Fault, and limited to the west by the slope-fault system. The study area evolved through the multiphased rifting process that ultimately originated the Northern Atlantic Ocean. This process started from the Late Triassic, until the continental breakup event between Iberia and Newfoundland, in the Aptian-Albian. From this moment onwards the margin was filled with the post-rift sedimentary cover, until the Late Cretaceous. During the Cenozoic the margin suffered a generalized tectonic inversion phase, which also affected the study area. There was differential subsidence and uplift troughout the defined sectors that can explain the less sedimentary cover on the continental shelf, and thicker packages beyond the continental slope. The Porto Basin’s lithostratigraphic chart was redefined taking in account previous publications and reinterpreting well reports, displaying a good deal of similarities to the Northern Lusitanian Basin. The wells intersected a few intervals that show some evidence for hydrocarbon generation and accumulation throughout the basin, and two main source rocks were identified. The first one was on the base of the Early Jurassic marine shales (base of the Esturjão Formation), and the second one on the Upper Jurassic marls (the Oxfordian Cabo Mondego Formation). These two intervals should match the ones identified on the adjacent Lusitanian Basin, for the Early Jurassic (Água de Madeiros and Vale das Fontes formations), and the Late Jurassic (Cabaços/Vale Verde Formation). Through the geochemical data analysis it was confirmed that the Early Jurassic source rock intervals held promising evidences for petroleum generation, especially the ones identified on the Lula-1 well. The Oxfordian source rock was only identified at the base of the Cavala-4 well, showing a fair potential for hydrocarbon generation. However, seismic lines interpretation lead to believe that this source interval shall be marginally distributed, controlled essentialy by paleotopographic features and pre-existing structures prior to the Oxfordian deposition. The tectono-stratigraphic analysis took in consideration some classic publications, such as Gawthorpe (1987), Ebinger (1999), Morley (1999), Leeder & Gawthorpe (2000) and Acocella et al. (2005). These works showed to be extremelly useful to recognize tectonic models at this margin sector from analogue rift systems. Based on these analogues, and interpreting thickness maps for each sismostratigraphic unit, it was possible to consider the existence of relay ramps during the rift climax, on the Late III Tectono-Estratigrafia e Modelação de Sistemas Petrolíferos da Bacia do Porto

Abstract

Jurassic and Early Cretaceous. These should be responsible for the establishment of essentially siliciclastic depositionals systems from a higher, proximal area, into the newly-formed distal zones, created by intense tectonic subsidence. During this phase the ENE-WSW accommodation zones should have evolved into transfer faults, controlling the rift shoulder sedimentary bypass into the distal zones, on the west. Thermal modeling exercises were performed using PetroMod 1D (IESSchlumberger) software, in order to evaluate maturity of the two main potentially generating intervals. The integration of organic geochemical data, kinetics and the establishment of boundary conditions (heat flow, paleowater-depth and surface-water interface temperature) lead to define the basin’s thermal evolution through time and its impact on the oil and gas generation windows. The results of thermal modeling showed the critical moment of hydrocarbon expulsion, which influenced the evaluation of petroleum systems timing of events. Considering the modeling outcomes and the evaluation data on some well reports, the type and character of petroleum system elements for Porto Basin was reevaluated, and a Petroleum System Events Chart was defined. In order to evaluate the presence and types of possible petroleum systems on the underexplored deep offshore included on the study area, lithostratigraphic knowledge, geodynamical evolution, transgression-regression cycles and sea-level variation were projected from Porto Basin into the distal area. This technique, commonly known as the «Wheeler Diagram», proved to be extremely useful as an insightful speculative exercise of lithology variation predictions on the proximal external margin, which has impact on petroleum system elements definition. The comparison of the deep offshore domain to the west of Porto Basin with the Canadian conjugate margin, namely the Flemish Pass Basin, was important as an exploration analogue. Recent commercial discoveries on that basin constituted an important exploration paradigm shift, and it’s expected that it could be a good prenounce for refocusing the petroleum exploration on this margin, which has a similar lithostratigraphic sequence.

Keywords: Porto Basin; Tectono-stratigraphy of rift-type basins; Organic geochemistry; Thermal modeling; Petroleum Systems

IV Tectono-Estratigrafia e Modelação de Sistemas Petrolíferos da Bacia do Porto