Abstract

An integrated multidisciplinary approach was effectively applied to a regional evaluation of the sedimentary basins of the Philippines The approach resulted in the production of a comprehensive exploration package which appreciably enhanced the understanding of the petroleum geology and hydrocarbon potential of the studied areas More significantly, potential of the studied areas More significantly, new exploration concepts were formulated which offer a host of possibilities to explorationists who will venture into the marginally explored basins of the country.

Introduction

In order to revitalize exploration interest in the country, the Philippine government, with partial funding from the World Bank, commenced the 2 year Petroleum Exploration Promotion Project in mid-1983. The project had three interrelated components, namely; a 9000 kilometer marine seismic program, a 216,000 kilometer aeromagnetic survey program, a 216,000 kilometer aeromagnetic survey and a countrywide basin evaluation program. The Philippine Bureau of Energy Development, with the guidance and assistance of Robertson Research (Australia) Pty. Ltd. and Flower Doery Buchan Pty. Ltd., undertook the basin evaluation program. The study aimed to:

  1. investigate the petroleum geology and assess the hydrocarbon prospects of 13 sedimentary basins (shown in Figure 1);

  2. rank or grade the basins according to petroleum potential; and

  3. assemble and promote an exploration package which incorporates the evaluation of the basins.

The basin evaluation was particularly unique for the Philippines on two counts: magnitude and scope, and the corollary promotional aspect Not only were the subject sedimentary basins numerous, they ranged from totally unexplored to marginally explored areas with extremely variable amounts of available data. The newly-acquired marine seismic and aeromagnetic records also extended the study to deepwater areas or slopes on most coastlines. Furthermore, it was not enough that the exploration package was geoscientifically valid based on modern basin-analysis techniques, it must also be presented in a format and standard geared to attract the attention of the international oil exploration industry. In view of the objective of the basin evaluation set forth, an approach was used which utilized and integrated all applicable disciplines and data, and whose product was tailored to meet the needs of the oil industry.

BASIN EVALUATIONS AND PREVIOUS EXPLORATION STRATEGIES

A review of the petroleum exploration track record in the Philippines shows that systematic basin studies and evaluations are fairly recent phenomena. Basin evaluations were generally very crude or unheard of from the period 1890-1972 when a total of 306 wells were drilled. Most of the wells were drilled to shallow depths based on limited geoscientific information. In fact, the earlier wells were sunk solely due to occurrences of nearby surface oil and gas seepages. A significant number of wells contributed little or no new information of regional or local value because of repeated drilling to shallow depths over the same structure. The strategy for this exploration phase basically involved 1) definition of structural traps - usually Miocene anticlinal folds - mapped by surface geology which were occasionally confirmed by geophysical surveys and 2) exploration or step-out drilling. The year 1973 was practically the start of modern oil and gas exploration in the country. Basin studies became common exploration techniques. However, such studies were of limited scope because of unavailability of exploration data beyond service contract areas and/or inadequate input of necessary expertise. The limited application of basin evaluation as an exploration tool is reflected in the statistics of prognosticated traps of exploratory wells for the prognosticated traps of exploratory wells for the period 1973 through 1984. Of 133 exploratory wells period 1973 through 1984. Of 133 exploratory wells drilled during this exploration phase, 67% attempted to teat carbonate reef stratigraphic traps, 3% turbidite stratigraphic traps and the rest, clastic reservoirs mainly in anticlinal structural traps. The dominance of carbonate reef targets may be partly attributed to the fact that these were more partly attributed to the fact that these were more easily defined by modern seismic, and the only production to date comes from carbonate reservoirs. production to date comes from carbonate reservoirs. However, it is clear that other potential trapping mechanisms had not been extensively considered, greatly limiting the number of potential plays tested to date.

DATA BASE

The available data used for the basin evaluation can be grouped into 'existing' and 'new' data. The existing data refers to data on hand at the Bureau of Energy Development (BED) and public or private entities at the start of the project. New data, on the other hand, includes all data acquired in connection with or within the project duration. The study is hence current up to approximately the end of 1985. The existing data can be further categorized into:

  1. Published and unpublished reports and papers of government, industry, research and academic institutions. These include amongst others: - Well Completion Reports and appended data such as lithologs, wireline logs and test results Geophysical Reports (seismic, gravity, magnetic), including structure maps, seismic record sections, velocity surveys and basement depth maps - Basin Study Reports of selected individual basins conducted by the BED and various research groups/consultants - Field Geological Reports and Regional Geological Studies - Scientific or Research Papers

  2. Geological Samples - Ditch cuttings, cores, oils, etc. The new data are mostly geophysical and laboratory data. These include: - 9000 kilometers of marine seismic profiles with corresponding marine gravity and marine magnetic records 216,000 kilometers of high resolution, detail grid aeromagnetic data covering two-thirds of the archipelago Geochemical Analyses of selected well and field samples and oils - Paleontological Analysis - Sedimentological Analyses The quality, and consequently, usefulness of each data set varied immensely within a single basin and from one individual basin to another.

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