How “Active” are Uplifted Petroleum Systems? A Discussion Based on Outcrop Observation and Modeling Results From the Lodève Basin (Southern France)
For an ideal low risk conventional prospect, the source rock should be at maximum burial, with an oil generation peak occurring well after the emplacement of the trap (which includes an effective reservoir and seal), but not too recently to allow for expulsion from the source rock and migration into the trap. However, numerous examples of oil and gas fields in tectonically active regions (e.g., Zagros fold and thrust belt) prove that charge and trap formation occurring simultaneously can result in large accumulations, often associated with important leakage. The Kimmeridge Oil field (Southern UK) is an anticlinal structure formed during Cenozoic Alpine compression which also triggered the uplift of the source rock, thus shutting down the “oil kitchen”. However, a lag in oil expulsion seems to be the most likely driver for a retarded charge, which might still be ongoing today, indicating a lag of several tens of millions of years between maximum burial (generation peak) and residual charge. The question, however, remains, whether such a system can also be proven for intense, large‐scale erosion and whether reservoirs which did not even exist at the time of maximum burial can be charged. In this study, the long‐lived petroleum system of the Lodève Basin (Southern France) is presented. Although not of great economic importance, the study area is ideally suited to model the structural, thermal and petroleum system evolution, because geological history is well‐constrained by field observations and laboratory measurements; this is rarely the case for offshore basins with deeply buried rocks and unconformities. Geometrical reconstruction of the syn‐rift stratigraphy and the geology in the north gives a good estimate of the missing Permian and Mesozoic strata, and thus the amount of uplift. The internal stratigraphy, organic richness and thermal maturity of Autunian black shales are well defined, and live oil seeps prove downward expulsion and fault bitumen upward fault migration. A small oil field in a neighboring Permian basin attests accumulation in Triassic reservoirs. The modeling results indicate that peak oil generation from the black shales has very likely been reached during Late Permian, and ceased with the uplift, without any noticeably effect from the following Mesozoic burial and uplift events. Despite the end of oil generation, modeling suggests that the petroleum system has been active over long periods after maximum burial. Even though the main expulsion occurred during peak generation, remnant oil expulsion from the metric shales into the internal sandstones is modeled to continue slowly until present day, with an acceleration due to recent erosion of the Mesozoic. As a result, the modeled present‐day oil saturation within the black shale and its internal sandstones shows higher values than directly after the Permian uplift. Mid‐ Triassic sandstones, which are locally in direct stratigraphic contact with the black shales but separated by the Base Triassic unconformity, are modeled to contain oil. Such accumulations of oil generated during Late Permian in Triassic reservoirs are proven. Their existence require that the retarded oil charge bridges a time gap of at least ~15 Ma between the peak oil generation, the uplift and erosion of ca. 1500m of Permian syn‐rift strata, and the deposition of the reservoir and seal.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019