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PETERS, K.E., Mobil Technology Company, J.W. SNEDDEN Mobil Exploration Norway, Inc., A. SULAEMAN, Mobil Oil Indonesia, J.F. SARG, and , R.J. ENRICO, Mobil Technology Company

Abstract: A New Geochemical-Sequence Stratigraphic Model for the Mahakam Delta and Makassar Slope, Kalimantan Indonesia

The accepted stratigraphic model for the Mahakam-Makassar area downgrades the potential for commercial deepwater hydrocarbons on the outer shelf and fails to explain recent oil and condensate discoveries at Perintis, Sisi, Ragat, and Merah Besar (Figure 1). According to this model, effective Middle Miocene coaly source rocks occur in updip shelf areas, while rocks of equivalent age on the outer shelf are (1) thermally postmature, and (2) too deep to retain good reservoir quality.

Our reinterpretation of seismic and sequence stratigraphic data for the outer shelf area suggests that the Middle Miocene source rock is not buried as deeply as originally believed and that much of it is now within the oil window. This revised geochemical-stratigraphic model upgrades the potential of the outer shelf and explains recent large oil discoveries in deep water (e.g., Merah Besar). The model predicts genetically distinct petroleum accumulations that are charged from local kitchens between anticlinal structural trends aligned parallel to the coastline. High-resolution geochemistry confirms that crude oil samples from these different trends (e.g., Handil-Nilam-Badak, Bekapai-Tunu-Attaka, Perintis-Sisi-Ragat) are genetically distinct.

Two major (1-2) and two minor (3-4) petroleum systems dominated by terrigenous type III organic matter are distinguished geochemically (Figure 1-2):

(1) Waxy "highstand" oils (46% of samples, e.g., Handil, Nilam) occur mainly onshore in Middle Miocene, but also in Upper Miocene and Pliocene reservoirs. These oils originated from Middle-Upper Miocene coals and shales deposited in nearshore coastal plain highstand kitchens that are now near the peak of the oil window.

(2) Less waxy "lowstand-1" oils (31%, e.g., Perinitis, Sisi, Ragat) occur offshore mainly in Upper, but also Middle Miocene reservoirs. These oils originated from Middle-Upper Miocene coaly rocks deposited in deepwater lowstand kitchens now mostly in the early oil window.

(3) "Lowstand-2" oils (15%, e.g., Semberah-037) are similar to the lowstand-1 oils, but occur mainly onshore in Middle or Upper Miocene reservoirs. These oils are more thermally mature than lowstand-1 oils and may have originated from Lower-Middle Miocene coaly rocks.

(4) Nonwaxy "transgressive" oils (8%, e.g., Badak) occur mainly onshore in Middle or Upper Miocene reservoirs. They originated at low levels of thermal maturity from Middle Miocene marine shales deposited near maximum flooding surfaces.

The new geochemical-stratigraphic model predicted that lowstand oil and gas rather than gas would be discovered at the recently drilled deepwater Merah Besar-6 well (Figure 2).   Chemometric analysis of data for oil samples from the well confirms that they belong in the lowstand oil group.

Source rocks for the highstand and lowstand oil groups were dominated by type III terrigenous organic matter deposited under oxic conditions. Source rocks for the transgressive oils contained significant marine organic matter, suggesting mixed type II/III kerogen deposited under suboxic conditions. Because of its location between the highstand and lowstand kitchens,Tunu Field received oil charges from both kitchens. Source rocks for the highstand and lowstand oils contain kerogens that generate hydrocarbons at slower rates than the classic type III Mahakam kerogen. Our thermal model for highstand and lowstand source rocks indicates that less fractional conversion of the kerogen occurred than predicted by the accepted stratigraphic model and classic type III kinetics. Furthermore, 2D fluid-flow modeling supports the geochemical evidence for commingling of oils in Tunu Field from kitchens to the east and west.

FIGURE 1. Map shows anticlinal trends (elliptical shapes) and locations of oil samples in the Mahakam-Makassar area, Kutei basin. Heavy and light stippling indicate mainly oil and gas trends, respectively. Genetic groups with symbols defined in the inset at lower right are based on chemometric analysis of multivariate geochemical data. Star indicates location of the first oil discovery in the Mahakam Delta in 1898.

FIGURE 2. Schematic of the new geochemical-stratigraphic model and predicted distribution of Mahakam-Makassar area source rocks along X-X' (Figure 1). Faults, which provide inferred migration pathways from source rocks to reservoirs, are not shown. HST = highstand systems tract,TST = transgressive systems tract, LST = lowstand systems tract, MFS = maximum flooding surface.

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England