The PESGB thank Ikon Science for sponsoring this London Evening Lecture wine reception

Seal breach, fluid expulsion and a mud volcano in East Java

Richard E. Swarbrick, GeoPressure Technology

Tuesday 9th September 2008, doors open 5.45 pm for 6 pm
NB. doors close 6.20 pm
 Burlington House, Geological Society, London

Introduction
Some valid traps have hydrocarbons present, some have not.  There are a number of possible explanations including lack of a mature source rock, late trap development and migration “shadow” as well as loss of hydrocarbons which enter the trap.  Loss of hydrocarbons through a seal may involve hydrocarbon buoyancy (membrane leakage) or more catastrophic events due to fracturing (hydraulic leakage) and cross fault seal failure.  The aim of the talk is to examine conditions for hydraulic leakage, using examples from the North Sea and Gulf of Mexico, and examine the origin of a mud volcano in East Java, Indonesia - seal breach or underground blow-out?

Hydraulic Failure
When pore fluid pressures reach the minimum stress plus tensile failure of their host rock, fracturing will occur.  Fractures will continue to propagate so long as the high pore fluid pressure is maintained.  Fracturing opens up high permeability conduits which allow hydrocarbons in a trap to remigrate, either to another reservoir or to the surface.  Regions of high pore pressure are therefore prone to seal breach.

Case Study: Gulf of Mexico
The Popeye and Genesis Fields, located in the Mississippi Canyon area of the Gulf of Mexico illustrate seal breach accompanied by sea floor leakage.  Seldon & Fleming (2005) describe the two fields located either side of a “mini-basin” but connected in fluid and pressure terms by laterally continuous reservoirs.  The reservoirs at the Genesis field are shallow.  MDT fluid pressures and LOT fracture pressures are similar, indicative of high seal breach risk.  Surface seeps above the field and two water-wet reservoirs testify to trap leakage.  At the deeper reservoirs in Popeye Field, by contrast, where the same excess pressure does not create a pressure regime close to the fracture gradient, and here significant hydrocarbon volumes are present in all reservoirs.  

Case Study: Central North Sea
Analysis of seal breach in the Central North Sea HPHT region was conducted by GPT / IHS as part of a regional pressure study.   A dataset of 67 wells ( 43 discoveries and 24 dry holes) was used.  The analysis at top reservoir shows 88% discovery rate when the pore fluid pressures (water-phase) are more than 1200psi (83 bar) greater than the minimum stress.  The discovery rate falls to 35% when this “aquifer seal capacity” is less than 1200psi.  An even clearer separation of dry holes and discoveries is found if the analysis is extended to BCU and chalk with some simplifying assumptions.  The analysis agrees with the identification of vertical fluid migration pathways through the Chalk seen in core in several wells.

Case Study: Mud Volcano, East Java
On May 29^th 2006, a mud volcano (known as “LUSI”) erupted in paddy fields, close to factories and housing at Sidoarja, a suburb of Surbaya, East Java, the second largest city in Indonesia.  The eruption was 150m from an exploration well which was in the process of handling a kick, but surface flow commenced only 2 days after the Yogyakarta earthquake, in South Java.  There are therefore two competing theories for the eruption: (1) drilling induced sub-surface to surface blow-out or (2) natural “breach” by an earthquake-induced pressure pulse.

The evidence on both sides will be assessed.  Whatever the cause, the mud volcano remains an environmental and social-economic disaster for the region.  More that 10,000 homes have been inundated and there remains the challenge of coping with approximately 10.0 MMBM/D (million barrels of mud per day).  The long term implications remain uncertain, but seismic examination of a pleisteocene “collapse feature” 5.0km away at Porong suggests a wide area will be affected by future subsidence / collapse.  We may be about to witness a rare geological phenomenon – the birth and early collapse history of a mud volcano, in this instance in a dense-populated onshore location.

Acknowledgements
IHS for pressure data used in the Central North Sea study;  Richard Davies, Mark Tingay and many others who have assisted with data and interpretations connected with the LUSI mud volcano.

Reference:
Seldon, B  & Flemings, P.B., 2005.  Reservoir pressure and seafloor venting: Predicting trap integrity in a Gulf of Mexico deepwater turbidite mini-basin.  AAPG Bulletin v. 89, p. 193-209

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