Outcrop examples of mudstone intrusions from the Jerudong anticline, Brunei Darussalam and inferences for hydrocarbon reservoirs

Abstract

AbstractMudstone intrusions in the Jerudong area represent natural hydraulic fractures developed above an inferred mobile mudstone diapir sourced from the Middle Miocene Setap Formation. Intrusions are manifest as dykes, sills and laccoliths. Intrusion geometries are strongly influenced by pre-existing weaknesses, in particular normal faults. Local inflation of country rock shale units also occurs. Dyke terminations include curved traces, splays and en echelon faults. The wall rocks tend to be smooth, or have numerous small, sub-horizontal ledges (mini jogs). Faults and bedding surfaces cause larger jogs in the dyke trend. Mudstone sills commonly change thickness at normal faults, and inflate the hanging wall so that the normal fault sense of motion is inverted, thrusting and fault bend folding are the result. An exposed mudstone laccolith passes laterally via v-shaped bedparallel intrusions into sandstones. The roof of the laccolith is arched upwards and near the intrusion, the roof is broken up by small and large intrusions. Existing models for diapir rise require the brittle shear strength of the roof sequence to be overcome. The Jerudong outcrops demonstrate diapir rise by hydraulic fracturing and stoping which limits the roof sequence strength to the minimum horizontal stress plus the tensile strength of the country rock or normal fault zones in the roof sequence. The crosscutting mudstone dykes, sills, mudstone intruded fault zones and associated cataclastic deformation form planar permeability barriers that can highly compartmentalize the reservoir rock and significantly reduce the area of reservoir rock that can be effectively produced of hydrocarbons. Intruded mudstones could easily be mistaken for stratified shales when interpreting and correlating well logs.