Somaliland: Causes of overpressure Oil Seeps

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Oil seeps in SomalilandBy Abdiwahab Ahmed Yassin Gabay Haddi

Somalilandsun -A petroleum seep is a place where natural liquid or gaseous hydrocarbons escape to the earth’s atmosphere and surface, normally under low pressure or flow. Seeps generally occur above either terrestrial or offshore petroleum accumulation structures. The hydrocarbons may escape along geological layers, or across them through fractures and fissures in the rock, or directly from an outcrop of oil-bearing rock.

A petroleum seep occurs as a result of the seal above the reservoir being breached, causing tertiary migration of hydrocarbons towards the surface under the influence of the associated buoyancy force. The seal is breached due to the effects of overpressure adding to the buoyancy force, overcoming the capillary resistance that initially kept the hydrocarbons sealed.

Causes of overpressure

The most common cause of overpressure is the rapid loading of fine-grained sediments preventing water from escaping fast enough to equalize the pressure of the overburden. If burial stops or slows, then excess pressure can equalize at a rate that is dependent on the permeability of the overlying and adjacent rocks. A secondary cause of overpressure is fluid expansion, due to changes in the volume of solid and/or fluid phases. Some examples include: aqua thermal pressuring (thermal expansion), clay dehydration reactions (such as anhydrite) and mineral transformation (such as kerogen to oil/gas and excess kerogen).

Types of seeps

There are two types of seep that can occur, depending on the degree of overpressure. Capillary failure can occur in moderate overpressure conditions, resulting in widespread but low intensity seepage until the overpressure equalizes and resealing occurs. In some cases, the moderate overpressure cannot be equalized because the pores in the rock are small so the displacement pressure, the pressure required to break the seal, is very high. If the overpressure continues to increase to the point that it overcomes the rock’s minimum stress and its tensile strength before overcoming the displacement pressure, then the rock will fracture, causing local and high intensity seepage until the pressure equalizes and the fractures close.

Somaliland Oil Seep

Odweyne has thousands of naturally occurring seeps. Much of the discovered in Somaliland during the 19th century was from observations of seeps. The Somaliland’s largest natural oil seepage is Xidhka Saliida Oil Point in the Dhagax Shabel Berbera at Dhaban Basin Nine of the better known oil seep locations in Odweyne are Ceeg and bodhley Pits. At Daad madheedh River Field, there are no active seeps. However, oil stained formations in outcrop remain from previously active seeps. Seep geochemistry indicates that light oil is being generated. The seeps lie on the margins of basins identified by the depth to basement study and close to faults interpreted on the Landsat imagery. Based on a regional overview, a tentative assessment of the reservoir and seal intervals indicates that suitable rocks could be present in Odweyne Basin.

Oil seeps in Somaliland

The initial interpretation of the structure of Block 26 combined with the known complexity of the regional tectonic history indicates that suitable structural trapping possibilities will be found. A preliminary examination of the satellite image revealed a possible roll-over anticline measuring 30 km x 10 km and with hydrocarbon seepage on both flanks. Nine oil seep samples were located By Abdiwahab A Yassin and analysed by Geochemical test in May 2005. The samples had been collected in the eastern part of Odweyne Basin on the northern and southern margins of the basin identified by the depth to basement study.

The geochemical test report suggested light oil seepage with persistent amounts of extractable hydrocarbons in all samples. The samples were biodegraded and the lightest fractions lost by evaporation. Biomarkers suggested a moderate to advanced source maturity. The low ratios of dibenzothiophane to phenanthrene suggest a clastic as opposed to carbonate source. The samples were not suitable for further source rock characterisation. There were no obvious terrestrial or lacustrine source markers although given the nature of the samples such sources cannot be ruled out.

These results are regarded as promising and they support the view that the basin revealed by geophysics supports an active petroleum system.

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