Seismic Stratigraphy Supervised Inversion: Exposing the Internal Features of Ngrayong Formation, East Java Basin

Abstract

The rise and fall of sea level changes during early to late Miocene in East Java Basin consequently influenced the stratigraphic variability and the depositional settings of Ngrayong Formation. Ngrayong Formation as a hydrocarbon prone formation in East Java Basin is a product of a comprehensive variety of depositional environments and a number of sedimentary developments. Three depositional units have been discovered which comprise of the stratigraphic Unit I & II that represent a facies change within the lower regressive part of Ngrayong Formation while Unit III represent upper transgressive part of the cycle and overlies the other two stratigraphic units. The concept of sequence stratigraphy brings the understanding of sedimentology towards knowing both in geometrical and scale of depositional element point of view. Stratigraphical features are well pronounced based on the well and seismic datasets revealing the depositional sequences of the Ngrayong Units. Golf-2 well-data sequence stratigraphy expressed nonetheless three sequences of system-tracts and its stacking patterns, respectively. While Golf-2 and November-3 well-data quite relate the same chronostratigraphic events, November-3 consist less system-tract and stacking pattern. The Ngrayong depositional sequence at November-3 comprises relatively thicker highstand system-tract and thinner transgressive system-tract on the top representing that Ngrayong Formation over Golf-2 and November-3 are positioned at different depositional settings. The well-data sequence stratigraphy quite correlates with the seismic stratigraphy which has been interpreted on each system-tracts are belonging to the regressive-transgressive-regressive sequence of Ngrayong Units. Well-data sequence stratigraphy exposes a relatively thick highstand system-tract towards transgressive system-tract on the top of Ngrayong Formation. The Ngrayong Formation comprise promising geometry interpretation. The reflection termination of the maximum flooding surface represents the boundary between the lower transgressive system-tract and the highstand system-tract which act as a downlapping horizon while the transgressive surface represents the boundary between the highstand system-tract and the transgressive system-tract which act as a toplapping horizon where the transgressive system-tract starts to onlap. A clinoform or foresets are apparent in the highstand system-tract with a sigmoidal progradational geometry concluding the depositional sequence are draping on low energy stance and indicate a suspension sediment supply. Seismic inversion is supervised by the system-tract boundaries to represent the stratigraphical features towards the initial model. Geological derived an initial model represent the base distribution of P-Impedance value which seemingly forms the sigmoidal patterns of the stratigraphy and representing the transgressive system-tract which has a relatively lower P-Impedance value towards the highstand system-tract. The seismic inversion resulting the final P-Impedance model with a great disseminated P-Impedance value within the distinctive depositional sequences. The comparison between supervised and unsupervised inverse model quite comprise promising results. The supervised inverse model exhibits each depositional sequence with fashioned pattern P-Impedance value while the unsupervised inverse model lack of detail. Clinoform geometries of Ngrayong Formation are clearly defined whereas conventional inversion framework merely non-unique geometry. The integration between seismic stratigraphy and seismic inversion is a valuable aid to greatly expose interesting internal formation features with a high variability of depositional setting. Our study point-out that a comparable formation characterized by a high stratigraphic variability suggested to follow the similar approach.

AAPG Datapages/Search and Discovery Article #90336 ©2018 AAPG Asia Pacific Region, The 4th AAPG/EAGE/MGS Myanmar Oil and Gas Conference, Myanmar: A Global Oil and Gas Hotspot: Unleashing the Petroleum Systems Potential, Yangon, Myanmar, November 13-15, 2018