Refinements of Stratigraphic Markers in Mak-Ban Geothermal Field Using Recent Gamma Ray Logs and Implications to Well Targeting

摘要

Gamma Ray (GR) Logging is widely used to measure the amount of gamma ray emission from sub-surface rocks as a consequence of radioactive decay of certain elements (i.e. Potassium (K), Uranium (U), and Thorium (Th)). In the Mak-Ban Geothermal Field, located in the provinces of Batangas and Laguna, Philippines, and operated by Philippine Geothermal Production Company, Inc., the GR log is used to identify three major stratigraphic markers: Spherulitic Rhyolite 1 (SRI), Spherulitic Rhyolite 2 (SR2) and the Andesite Lava Marker (ALM). These markers play an important role in well targeting and reservoir modeling. The SRI and SR2 units, known to be primary sources of permeability in the shallow reservoir, have distinctive high GR signatures. On the contrary, the ALM, which is believed to be a low permeability unit that marks the separation between shallow and deep reservoirs, is associated with low GR measurement. Previously, well correlation and geologic modeling of the aforementioned stratigraphic markers have been challenging due to the limited GR data from runs conducted during drilling of only a few wells. However, recently acquired GR log data executed simultaneous with routine downhole surveys of existing wells, complemented by other subsurface information, has enabled improvements to be made to the correlation of these units and to the current geologic model. Results show that the general depths and thickness of the markers differ in each sector of the field. For example, the central sector has relatively shallower markers compared to the north sector and far southeast sector. The presence of faults may be a major factor that contribute to the observed differences in depth. The implications of these recent findings to well targeting and casing programs are also evaluated as preparations are underway to resume make-up well drilling at Mak-Ban.