The Penyu Basin Revisited: The Abandoned ‘Mate’ of The Malay-Natuna Basin

The Penyu basin is a transtensional-transpressional basin that developed approximately coeval to the greater Malay Basin to the north. In spite of the stratigraphic and structural similarities of these basins, Penyu basin has had marginal results in terms of discovered volumes of hydrocarbons, and no commercial discoveries yet made on the Malaysian side of the basin. Conventional industry wisdom has attributed this largely to source rock leanness, most likely consisting of lacustrine-type sediments as isolated pods in the deeper portions of half grabens. Poor drilling results since the early 70’s and the elusive nature of this uncalibrated source rock has kept explorers out of Penyu in recent years. Nevertheless, straddling the Malay- Penyu basin is the largest field discovered thus far: the ~350-400 mmbo Belida field. Belida is distinctive in many ways: it consists of a mildly inverted structure sitting on a basement ridge that separates Malay from Penyu basin, it is not underpinned by source rock, thus relying on long distance migration, and has a distinctive oil signature that can be linked to a possibly significant contribution from a Penyu source. In addition, the Rhu oil discovery indicates that there is a working petroleum system within Penyu itself. Post drill analysis, 3D maturation-migration modeling and detailed structural geology suggests that drilling failure in the Penyu basin may be attributable primarily to the following reasons: structural timing versus peak HC generation, trap preservation (especially on the Indonesian side of the basin), trap definition -not one single well has been drilled using 3D data- and migration- given the likely anisotropic character of the carrier beds. Therefore, even though Penyu Basin has a seemingly less abundant petroleum system than the Malay basin, we sense that of all the perceived risks outlined above, only source rock presence and quality may not be derisked ahead of the drill bit. From our regional studies, there are a multiplicity of plays to be tested independently; and the use of 3D seismic as a key exploration tool is required to test once and for all the prospectivity of the Penyu basin.