Permeability heterogeneity within the Jerudong Formation: an outcrop analogue for subsurface Miocene reservoirs in Brunei

Abstract

Abstract The Jerudong Formation (early Tortonian) cropping out at Punyit Beach (Negara Brunei Darussalam) comprises a heterolithic succession of deltaic strata. Probepermeameter data have been used to evaluate the spatial nature of permeability heterogeneity within shoreface, lagoon and distributary channel environments. Strata at outcrop comprise the same lithologies as those at depth in the abandoned Jerudong Field, and provide an analogue for parts of the Jerudong Formation and equivalents in other Bruneian oilfields. Identified permeability heterogeneities are associated with contrasts of up to three orders of magnitude, occurring at facies, genetic facies unit, bed-set, bed and laminar scales. Distributary-channel facies display the best outcrop permeability (>7000 mD). They include trough cross-stratified fills of relatively straight (often tidal) distributaries, and laterally accreted point bar sandstones deposited within meandering channels. These facies display upward decreasing permeability trends. Shoreface facies may also display high (>6000 mD) permeability, but are characterized by upward increasing permeability and are significantly more heterogeneous than channel facies. Lagoon fill successions are highly heterolithic, comprising sandstones, siltstones and mudstones with a wide range of permeabilities. 5 cm spaced outcrop permeability data are more variable, and display ‘average’ permeabilities several orders of magnitude larger than occur within similar facies encountered in the subsurface. The spatial scale, frequency and magnitude of permeability variation within the Jerudong Formation indicate that facies packages observed at outcrop are of sufficient scale (4–10 m thickness) to provide potentially useful scaled elements for flow simulation. A model using 0.25 m scale grid blocks has allowed assessment of intra-facies permeability variation (associated with bedding/cross-bed sets) upon recovery within different channel fills. The model predicts both trough cross-stratified and laterally accreted channel fills are heterogeneous with respect to waterflood, and preferential imbibition into their lower parts. Trough cross-stratified channels flood more rapidly than laterally accreted channel fills, where more fingering of the flood front occurs. Lagoon fill deposits display variable waterflood characteristics dependent upon sand content. Within both simulations, heterolithic lagoon fill facies appear to effectively compartmentalise modelled reservoir successions with respect to vertical cross flow.