--> ABSTRACT: Importance of "Basement" and Pre-Miocene Reservoirs in Southeast Asia, by Mark W. Longman; #91020 (1995).

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Importance of "Basement" and Pre-Miocene Reservoirs in Southeast Asia

Mark W. Longman

Most of the oil and gas produced in Southeast Asia comes from Miocene reservoirs, but exploration is increasingly focusing on the pre-Miocene rocks. Factors enhancing the potential for Pre-Miocene reservoirs include: 1) the common Paleogene source rocks in many Southeast Asian basins; 2) the complex tectonic history of the region which commonly resulted in the juxtaposition of basement blocks against the Paleogene source rocks; 3) the diverse depositional environments created by the onset of rifting around the margins of the Sunda Craton during the Paleogene; 4) and fairly limited exploration in the pre-Miocene rocks to date. Particularly important targets are pre-Tertiary "basement" rocks in highly faulted areas such as occur offshore from Vietnam, and Eocene sedimentary rocks around the margins of the Sunda Craton.

Five types of pre-Miocene reservoirs occur in Southeast Asia. These include: 1) structurally high "basement" fault blocks with granitic or metamorphic rocks that have been faulted, fractured, or weathered to enhance porosity; 2) Paleozoic and Mesozoic sedimentary rocks in stable intracratonic basins such as the Khorat Basin of Thailand, or in high fault blocks; 3) Paleogene volcanic rocks such as those producing oil at Jatibarang Field in northwest Java; 4) Eocene and Lower Oligocene sedimentary rocks deposited around the margins of the Sunda Shield; and 5) Upper Oligocene sandstones and carbonates lying immediately below the Miocene section, particularly in the back-arc and rift margin basins of the region.

Complicating the discovery of hydrocarbon accumulations in "basement" reservoirs is the fact that hydrocarbons do not necessarily occur at the very top of the basement section. Weathering prior to the onset of Tertiary deposition and sediment infiltration commonly filled all pores and fractures at the top of the basement section with shale or kaolinite and other cements. Thus, basement wells should be drilled at least 50 meters into the basement rocks to adequately test for hydrocarbon accumulations that might exist below a capping impermeable zone. It is also important to note that oil shows observed during drilling in weathered and/or fractured basement rocks may be subtle.

Although it is sometimes difficult to justify drilling a well with basement rocks as the primary objective, many prospects with shallower objectives can economically be drilled to basement. However, such wells do not necessarily test basement rocks in the most favorable position.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995