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

Abstract

Igneous sills emplaced at shallow levels in sedimentary basins commonly uplift the overburden and free surface. Uplift produces dome-shaped forced folds that may host economic hydrocarbon accumulations. These intrusion-induced forced folds are typically assumed to develop instantaneously, whereby the oldest 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 geologic timescales, with additional space-making mechanisms (e.g., compaction) partly accommodating 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 northwest Australia. Analyzing the seismic stratigraphy of the forced fold overburden allows us to recognize several distinct phases of fold growth. Subhorizontal reflections onlapping onto the lower portion of the forced folds at a high angle indicate that the first phase of sill emplacement and fold development occurred rapidly, facilitated by normal faulting, prior to the deposition of overlying strata during a period of magmatic quiescence and regional hydrocarbon maturation in the Early Cretaceous. Renewed magmatic activity resulted in a final, protracted phase of doming, which is recorded by a package of onlapping growth strata that was incrementally deformed by successive intrusive pulses. We also demonstrate that in addition to folding and faulting, the magma volume was likely accommodated by porosity reduction within the folded strata. Our observations imply that the age of the lowermost onlapping reflections only constrain 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 onlapping reflections during hydrocarbon exploration can thus provide critical insights into the potential volume and charge history of any hydrocarbon accumulations.