Decoding sill emplacement and forced fold growth in the Exmouth Sub-basin, offshore NW Australia: implications for hydrocarbon exploration

Abstract

Igneous sills emplaced at shallow-levels in sedimen tary basins commonly uplift the overburden and free surface. Uplift produces dome-s haped forced folds that may host economic hydrocarbon accumulations. These intrusion -induced forced folds are typically assumed to develop instantaneously, whereby the old est onlapping strata constrain the age of sill emplacement, and accommodate the entire volume of intruded magma. However, several studies demonstrate that forced folds may grow over geological timescales, with additional space-making mechanisms (e.g., compaction) partly a ccommodating the magma volume. It is thus critical to understand when forced fold traps form and how they evolve in relation to the timing of source rock maturation and migration. We analyze two forced folds imaged in 2D seismic reflection data from offshore NW Australia. Analyzing the seismic stratigraphy of the forced fold overburden allows us to recognize sever al distinct phases of fold growth. Sub- horizontal reflections onlapping onto the lower por tion of the forced folds at a high angle indicate that the first phase of sill emplacement a nd fold development occurred rapidly, facilitated by normal faulting, prior to deposition of overlying strata during a period of magmatic quiescence and regional hydrocarbon matura tion in the Early Cretaceous. Renewed magmatic activity resulted in a final, protracted p hase of doming, which is recorded by a package of onlapping growth strata that was increme ntally deformed by successive intrusive pulses. We also demonstrate that in addition to fol ding and faulting, the magma volume was likely accommodated by porosity within the folded s trata. Our observations imply that the age of the lowermost onlapping reflections only con strain the onset of sill emplacement and not the duration of magmatic activity. Constraining the dynamic evolution of intrusion- induced forced folds from the structure of onlappin g reflections during hydrocarbon exploration can thus provide critical insights into the potential volume and charge history of any hydrocarbon accumulations.