SWARBRICK, RICHARD E., GeoPOP, University of Durham, UK

Abstract: A Research Project Documenting the Pressure History of an Overpressured Basin - Application to Deep Water Pressure

Introduction

Knowledge of the magnitude of overpressure, the depth to the top of overpressure and the rate of pressure increase is required for safe and cost effective drilling everywhere. Deep water areas, drilled by conventional means using a mud-filled riser, encounter particular problems associated with well control when the pore pressure and the fracture gradient are close. Detailed experiences of drilling deepwater wells remain limited and overpressure is one of the biggest challenges, contributing to high cost wells.

Lessons can be learnt about expected overpressure magnitude and distribution from study of high pressure basins at conventional offshore water depths, and the techniques applied to the new deep water exploration areas. Compilation and display of all measured pore pressure and Leak Off Pressure (LOP) data is a necessary start. Examination of single and multi-well pressure vs. depth plots coupled with geological information (lithology, structure, faults, sedimentology) provides insights into the origin of overpressure and control of its distribution by lateral and vertical seals.

The Gulf of Mexico

Central North Sea and Delaware Basins will be used as examples where Compilations of data can be used to assess magnitude and distribution of overpressure. These examples build confidence in determining the impact of overpressure on deep water exploration.

Gulf of Mexico (offshore Louisiana)

Rapid sedimentation of up to 2.0 km/My in the Gulf of Mexico, offshore Louisiana, has led to siliciclastic sediment thickness above the Jurassic salt in excess of 10.0 km associated with the present day and ancestral Mississippi River and its delta. The parts of the sedimentary sequences dominated by fine-grained sediments, in particular the delta slope and pro-delta facies, are characterized by overpressures which increase in magnitude with depth. Overpressure can commence at depths of only 0.5 to 1.0 km burial, although where sandstones and siltstones dominate in the upper section (delta top/front facies) the depth to top overpressure is much greater. If the top of overpressure is deep and controlled by lithological change, there is usually a sharp pressure transition zone. Overpressure beneath the transition zone, throughout all fine-grained lithologies, increases approximately equal to the rate of addition of overburden (i.e. 12.0 to 12.6 MPa/km; 0.95 to 1.0 psi/ft). These observations are consistent with an origin from vertical stress loading (disequilibrium compaction). Departures from the regional pattern of increasing overpressure with depth derive from transfer of fluids and pressures due to lateral and vertical connectivity of sand bodies.

The Gulf of Mexico (offshore Louisiana) is representative of many Tertiary and modern delta regions including Mahakam Delta, Indonesia;Trinidad; South Caspian Sea; Niger Delta, offshore Nigeria and Nile Delta, Egypt. The top of overpressure commences either within a fine-grained sequence at about 0.5 1.0 km (depending on sediment properties and rate of deposition), or at a sharper transition zone determined by a lithology change. Overpressure.magnitudes at depths of 4.5 to 5.0 km can be as high as 50 MPa (7250 psi) without resort to any other mechanism than vertical stress loading.

Central North Sea

The Central North Sea Basin has overpressure within Tertiary and older sediments, including both siliciclastic and carbonate rocks. The principal basin architecture was established in Triassic/Jurassic times with rifting, after which time there has been progressive subsidence, driven in part by thermal relaxation. The most rapid phases of subsidence and associated sedimentation have been during late Cretaceous/Early Tertiary (90 to 50 Ma) and late Tertiary (5 to 0 Ma) times when sedimentation rates of up to 500 m/My have been experienced locally. Average sedimentation rates for the last 120 My range from about 35-55 m/My. Jurassic source rocks below about 4.0 km are gas generative.

The highest magnitude overpressures are found in the Jurassic/Triassic sandstone reservoirs with pressures close to the fracture pressure gradient below 4.5 km (14,000 feet). The origin of overpressure here is controversial.Vertical stress loading does not offer a full explanation for the very high magnitude overpressure. Some authors (Darby et al, 1996; Holm, 1998) suggest that gas generation is the primary cause. Swarbrick, et al (1998), however, argue that gas generation can only be a secondary cause of overpressure. The magnitude of overpressure from gas generation is critically dependent on the rate of volume expansion plus the connected aquifer volume, and calculations for typical North Sea pressure compartments suggest the magnitude overpressure from in-situ gas cracking in reservoirs is unlikely to exceed 2.1 MPa (300 psi).

The Cretaceous chalk is both a reservoir and a seal to both hydrocarbons and overpressure in deeper reservoirs. Pressure data from permeable units within the chalk are frequently above hydrostatic, except in rare situations where there is a hydraulic link with overlying normally pressured Paleocene sandstone reservoirs.The origin of overpressure in the chalk is not confirmed. One explanation, favored by Japsen (1998) using velocity-depth relationships, is disequilibrium compaction during late Cretaceous/early Tertiary and late Tertiary burial phases. Uncertainty results from poor knowledge of the timing of the cementation which has reduced the non-reservoir chalk section to its very low porosity today.Transmission of fluids and pressures from deeper in the stratigraphy, partly facilitated by fractures, is another possibility.

Overpressure distribution in Paleocene reservoir sandstones is controlled by the hydraulic connectivity of a regional sand sheet. High overpressure is present only where the sands are thin and isolated within overpressured Tertiary shales in the southeast of the basin. Elsewhere sands are near normally pressured due to lateral transfer into shallow subcrop areas in the northwest of the basin. An origin of overpressure by vertical stress loading is consistent with the continuous deposition of fine-grained lithologies above the Paleocene (and Eocene) reservoirs. The fluid retention depth is generally located between 0.8 and 1.4 km depth, based on wireline log response.

Delaware Basin, USA

Overpressure in the Delaware Basin is located in a dominantly fine-grained siliciclastic sequence sandwiched between normally pressured carbonates and clastics with associated evaporites above, and normally pressured fractured carbonates beneath. Rapid burial took place during Permian times (about 250 My ago) followed by very little additional subsidence (Luo et al, 1994). It is likely that vertical loading at that time created overpressure. Retention of the overpressure during the long time since rapid burial suggests extremely low permeability (< 0.01 nannoDarcy in the seal. The evaporites of the Castille Formation may have contributed to the effectiveness of the seal. Origin of overpressure by other mechanisms seems unlikely since there has been little additional burial and heating since Permian times.

Application to deep water areas

Vertical stress loading is the dominant mechanism in creating regional overpressure in reservoirs within geologically young basins. The principal factors governing the magnitude and distribution of overpressure by this mechanism are sediment burial rate and permeability evolution of the fine-grained lithologies, plus sediment geometry and associated reservoir connectivity.The main control on permeability is clay content (Yang & Aplin, 1998). In deep water areas the main exploration focus is on sandstone reservoirs deposited as turbidites with a high volume of associated fine-grained sediments, dominantly shales. The clay content of the fine-grained lithologies is likely to be high. In addition the top of overpressure is shallow when the deposition rate is high. Both these factors create conditions for high magnitude over-pressure with an increase in overpressure close to the overburden gradient.

Where there is lateral connectivity, especially with a turbidite fan geometry and large downdip communication with overpressured shales, the extra overpressure from lateral transfer effects are great. Deep water exploration areas are often dominated by fine-grained lithologies enveloping lowstand sandstone reservoirs, and shallow top of overpressure is to be expected where there is a high volume of clay-rich lithologies. Prediction using normal compaction trends is made particularly difficult because shallow overpressure and absence of logs near surface make it difficult to establish any of these trends on which overpressure magnitude can be estimated. In these circumstances forward modeling of pressure can provide valuable insights.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England