Burial Hydrodynamics and Subtle Hydrocarbon Trap Evaluation: from the Mahakam Delta to the South Caspian Sea
Authors: Grosjean Yves , Zaugg Patrick , Gaulier Jean Michel 
Published: May 2016

Abstract
The numerous highly accurate wire line pressure measurements taken in exploration and appraisal wells drilled in the Offshore Mahakam Delta PSC area during 1982 - 1991, combined with carefully calibrated shale pore pressure estimates and a vastly improved understanding of the stratigraphy, sedimentology and tectonic evolution of the Kutei basin (East Kalimantan, Indonesia), have allowed to propose a subtle trapping model that we call "Burial Hydrodynamic trapping". Water expelled by the very thick shale dominated sedimentary pile as it is rapidly buried, is drained laterally along higher permeability deltaic strata and causes hydrocarbon water contacts to be tilted, thereby offsetting the core of the accumulations from the anticlinal crests. This trapping configuration was found to be very common in the area of interest.
The model has been used in the first place to guide appraisal drilling during the evaluation phase of the Peciko discovery and Tunu western flank extensions. Trap closure maps were drawn using concepts and theory published by King M. Hubbert (1953, 1967). The well results demonstrated very quickly the validity of the model (Grosjean, 1994). Potential hydrocarbon traps by-passed by earlier exploration were then actively sought after in the Mahakam area, with among others two very clear successes at Stupa and South Tunu extension.
Meanwhile, an extensive review of other sedimentary basins of the world revealed that favourable and possibly similar conditions had actually been observed and described previously by various authors. In most if not all instances however, the available data would not allow the unequivocal identification of the lateral - and very slow - flow of water. The necessary multiple and accurate aquifer pressure measurements were missing, so that trap closure maps could not be drawn. The dynamic nature of the trapping mechanism was frequently overlooked by the authors, and observed aquifer pressure potential differences were sometimes interpreted as measurement errors, or more frequently as so called "perched water" conditions related to U-tube reservoirs isolated through faults or stratigraphy.
A critical and systematic review of the fields where the Company held a participating interest led to identify very clearly several cases of Burial Hydrodynamic trapping, in various basins, although with variations related to the specific geologic settings, and especially the faulting. The application of the model generally allowed deriving a more optimistic view of risks related to compartmentalization. The Smørbukk field, offshore mid Norway, and Mungo field, in the UK Central North Sea, are two good examples of such traps affected by the lateral drainage of excess water generated during burial. More recently published documentation has provided further examples of similarly trapped fields, with detailed descriptions of their pressure configurations: Britannia in the UK Central North Sea, Ormen Lange in the Norwegian Sea (O'Connor and Swarbrick, 2008).
At the exploration scale, regional maps can be drawn to provide a more accurate description of potential trapping conditions than depth structure maps only in laterally drained basins. Such a map was established for the South Caspian Sea area, which led to identify that the Absheron structure had most likely not been properly tested by the (unsuccessful) exploration well drilled in 2001.

Introduction
Subtle traps resulting from hydrodynamic conditions encountered in petroleum basins were described by several authors more than 50 years ago (see reviews and theory in Hubbert, 1953, 1967, and Hill, 1961). In a way, this was an outcome of the debate between the anticlinal and the hydraulic theories of oil and gas accumulation which was initiated with the expansion of hydrocarbon exploration towards the end of the 19th century (Munn, 1909). This debate was truly closed by King M. Hubbert in 1953, when he published his unifying theory of "Entrapment of Petroleum under Hydrodynamic Conditions".