Integrated Structural Restoration, Basin Modeling and Geochemical Analysis in Evaluating the Petroleum System Adjacent to Nansen Field, Western Gulf of Mexico
We demonstrate the strength of an integrated structural restoration and basin modeling workflow in a case study of Nansen field, Western Gulf of Mexico. Conventional basin and petroleum system modeling (BPSM) uses the vertical back‐stripping approach to describe the structural evolution of a basin. However, BPSM must be performed on structurally‐restored models, where lateral rock movement along faults or salt emplacement is significant in complex tectonic regimes. In this workflow, the structural evolution and basin thermal history are modeled in a “coupled” way: paleo‐restoration provides the overall structural evolution based on the assumption of hydrostatic de‐compaction, while basin modeling (PetroModTM) explicitly simulates the other geologic processes, such as thermal and pressure evolution, source maturity, hydrocarbon migration and accumulation. Seismic data, newly‐reprocessed by Anadarko in 2016, has greatly improved the sub‐salt imaging and illumination in the Deep Nansen area, which allows for a better mapping of the sub‐salt structural trap, trap timing, reservoir/source rocks and allows for a better understanding of the regional petroleum system and exploration prospectivity in this area. The PetroMod2D model that we built for the greater Nansen area is fully calibrated with abundant geochemical, fluid and PVT data available from the adjacent shallow Nansen and Boomvang fields. Quantitative diamondoid analysis suggests that shallow Nansen has received two‐stage hydrocarbon charges (mixing of cracked‐oil with black oil). The produced gas isotopes from shallow Nansen show overall high maturity gas (~1.2‐1.5% Ro) with a moderate mixture of microbial methane, indicating that some hydrocarbons have been biodegraded in reservoir due to its low present‐day temperature (less than 60 Celsius). However, the degree of biodegradation on fluids in the shallow Nansen Pliocene reservoirs is likely reduced by both hyper‐saline formation water and recent hydrocarbon charge. We test two‐end member hydrocarbon migration scenarios to charge shallow Nansen field: (1) shallow Nansen was charged by vertical hydrocarbon leakage from Deep Nansen; and (2) shallow Nansen was charged by the deep “mega‐basin” to the southeast. The second scenario is supported by several lines of evidence. (i) The presence of oleanane biomarkers in oil recovered from Deep Nansen fluid inclusion suggests the included oil is probably self‐sourced in the Wilcox interval. This observation is in alignment with the thick Mesozoic and Paleogene section mapped below the salt canopy at Deep Nansen. The thick Mesozoic/Paleogene section at Deep Nansen may have driven early HC expulsion from the postulated Jurassic source prior to the formation of the Deep Nansen structural trap and the emplacement of the salt canopy in Eocene; (ii) The Deep Nansen Wilcox reservoir pressure is extremely high, close to the regional leak‐off test (LOT) and fracture pressure trend. The extremely over‐pressured subsalt section in Deep Nansen supports the lack of hydraulic communication between shallow and deep Nansen; (iii) The modelled source maturity in the mega‐basin fetch agrees very well with that inferred from the produced gas isotopes of shallow Nansen. In addition, repeated high‐confidence oil slicks are observed to the southwest of this “mega” mini‐basin along the flanks of rising salt stock, which is envisioned as a hydrocarbon migration pathway.
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