New Approaches for Geological Uncertainties Handling in Reserves Estimation for Greenfield in South Vietnam Shelf

Abstract

Beluga oilfield discovered in 2014 locates in Southern part offshore of Vietnam –Cuulong Basin. Geological settings characterize by high oil column (1000m), complex faulting structure with amplitudes up to 500m, high lateral and vertical heterogeneity in reservoir continuation. Initially it was obvious that the Oilfield needs non-standard approach. To the very important but still common task of data collection and study which leads to accurate digital representation of reservoir geology, we added complex conceptual sedimentological modelling to ensure capture of geological uncertainties and their influence on final reserves calculation. All the data were thoroughly examined. There are 3 exploration wells drilled, from which well logs, core and well tests, mud reports, geological well reports, checkshots and 3D seismic cube over the area of interest are available. Well tests proved oil bearing reservoirs in Lower Miocene and Upper Oligocene, oil flows estimated at 7-260m3/d in Low Miocene and 191-961m3/d in Upper Oligocene. As well tests cover only 50% of whole net pay intervals, we categorized tested zones into four types of pay – good flowing, flowing, weak flowing and no flow. First 3 are considered as reservoir in further modelling. Miocene and Oligocene reservoirs were deposited in continental lacustrine and alluvial plain environments. As there was not enough core data to describe all reservoirs, we used electrofacies based on Log response shapes. We recognized Channel, Bar and alluvial plain facies. Due to the lack of RCAL data, it was not possible to get poroperm correlation for each facies, however later this was handled by geometry variations. Facies modelling was performed as object based modelling of electrofacies. Characteristics of geobodies geometry were chosen based on conceptual model and present day analogues. Three qualitied types of reservoir (good, poor, bad) based on Vsh analysis as well as from production logging data were modelled inside geobodies. For not tested reservoir NTG values from tested intervals were assigned. Structure, Net Volume, Types of reservoir, Water Oil Contacts and NTG values included in the uncertainty analysis. Reserves estimated at low (pessimistic) – twice less than base case and high (optimistic) – triple times more than base case. This extremely wide range of reserves reflects high geological uncertainty.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018