A precise science

Daniel Hitzman, President of Geo-Microbial Technologies (GMT) discusses the key issues with hydrocarbon microseep.

What exactly is a microseep?
Daniel Hitzman. Very small and very buoyant molecules of light hydrocarbon gases – methane, ethane, propane, butane and C5+ – escape from oil and gas reservoirs and penetrate their permeable reservoir seals. These gases travel vertically through micro fractures to the surface atmosphere. There are many seismic data sets that confirm such gas chimneys. These direct indicator gases can be measured at the surface in both soil and offshore sediment samples.

How do seeps help with oil and gas exploration?
Daniel Hitzman. Observing visible oil seeps at the surface – like outcrop stains or oil slicks on water – builds huge confidence that hydrocarbons are present in a basin or block area. Historically, parts of many Middle East basins were first targeted because of these visible surface macroseeps. But macroseeps reveal only part of the seepage spectrum story. Hydrocarbon microseeps of light hydrocarbon gases are now being used to more accurately locate oil and gas reservoirs at depth. These microseep gases are invisible to the human eye, but to gas chromatographs and microbial cultures they are clearly discernible and are very potent indicators of hidden traps charged with hydrocarbons.
 
Why test for hydrocarbon microseepage?
DH. Seismic data is imperative for making structural interpretations. Unfortunately, dry holes tell us that not all seismic traps contain hydrocarbons. We still drill an unacceptable number of dry holes. We must improve our exploration accuracy and optimise our development programmes. Microseepage surveys are great for ranking seismic prospects. I believe detailed microseep signatures reflect original reservoir heterogeneities before drilling and also accurately track dynamic reservoir pressures as production matures. Plus, as structures are becoming increasingly scarce, we must look for more subtle stratigraphic traps as future reserve builders. I think microseepage surveys may turn out to be the preferred exploration tool for discovering strat reservoirs.
 
What are typical microseepage survey strategies?
DH. Microseep surveys are very flexible in their design and scope. This translates to great efficiency and low cost. Microseep surveys commonly follow seismic programs and rank their numerous structural prospects. These post-seismic surveys help focus limited drilling budgets and avoid wasteful dry holes. In MENA basins we also frequently conduct large-scale reconnaissance surveys – sampling expansive frontier areas prior to expensive seismic shoots. In mature producing areas we utilise more detailed sampling patterns to locate by-passed reservoir compartments, offset well locations, and help plan waterflood and in-fill drilling projects.

How does GMT measure microseepage? 
DH. GMT conducts hydrocarbon microseep surveys throughout the Middle East and North Africa – both on and offshore. We collect shallow soil and sediment samples and then use two hydrocarbon microseepage detection methods: the Microbial Oil Survey Technique (MOST) and Sorbed Soil Gas (SSG). These microseep tools were first investigated by Phillips Petroleum Company and then further developed by GMT. For over 25 years we have worked with smaller independents, integrated majors, and national oil companies (NOCs). A MOST survey tests for specific microbes which consume thermogenic microseep gases. These microbial signatures are very reliable and reproducible and serve as excellent vertical seep locators. To distinguish whether the reservoir source is oil or gas or condensate, we supplement the microbial measurements with Sorbed Soil Gas (SSG) analyses. These SSG tests are conducted on leftover and preserved MOST samples or from separate and sometimes deeper samples. SSG results are measured in parts per million and the gas ratios are excellent indications of reservoir source. We find the combination of microbial and sorbed soil gas surveys to be an extremely cost-effective exploration strategy.
 
What does the future hold for GMT and microseepage advancement?
DH. As exploration budgets are stretched and tolerance for dry holes is rejected, the adoption of low-cost microseep surveys will obviously increase drilling accuracy and efficiency. Additionally, the ability to discover stratigraphic reservoirs and optimise mature fields will boost microseep acceptance. GMT looks forward to being a leader of microseep technologies and continuing our service to the petroleum industry.

For more information, please visit www.gmtgeochem.com or email danielh@gmtgeochem.com.