Geometrical Modeling of Reservoir Sandstones in Miocene Mahakam Delta, Indonesia

Roy D. Nurmi, Katsuei Saito, Takanori Uchiyama

The sandstone reservoirs within Tertiary deltaic sequences generally have limited areal extent. The geologic data from closely spaced, shallow wells in the Miocene deposits of the Mahakam delta were studied in order to develop a methodology for integrating local knowledge, core data, logs, and seismic data for the prediction and determination of reservoir geometry.

Seismic, dipmeter, and structural mapping were used for the structural evaluation of the drilled area, including the analysis of faulting. The determination of the structure penetrated by each well was also critical to the stratigraphic correlations and depositional analysis of these fluviodeltaic sandstones. The one oil-productive sandstone was determined to be a combination structural/stratigraphic trap across an anticlinal nose.

Thin coals and a few transgressive limestones, valuable as correlation markers, were used in unraveling the stratigraphic sequence with each sandstone present being correlated and individually mapped. The close well spacing permitted a reasonable definition of geometry and/or orientation for nearly every sandstone more than 5 ft thick. Most of the thin sandstones were considered to be crevasse-splay deposits with limited areal extent. However, nearly all of the reservoir-quality sandstones were identified as distributary-channel deposits, with a few bars of uncertain origin.

Eleven individual channels were defined and mapped within the study area. It was found that the bedding characteristics and paleocurrent data from individual wells could be used to determine the orientation of these channel reservoirs. Furthermore, the sedimentologic and paleontologic analysis of sidewall cores and nearby outcrops supported the overall depositional analysis. The study also provided insight into the thickness to width relationship of distributary channels in the Miocene Mahakam delta: width is approximately 25 times maximum thickness.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.