A scientific approach

By Daniel Hitzman

Searching for oil and gas deep underground requires discreet detective work: sifting through all the clues and data must be painstakingly and carefully considered. And even then it might be the very smallest molecule of evidence that is critical. That is the state of petroleum exploration and development today - sometimes the very smallest molecules are the key to guarantee success.

“The combination of microbial and sorbed soil gas surveys is the strongest, most integrated and cost-effective microseepage exploration strategy available”
-Daniel Hitzman

Micro is the new macro

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 MENA basins were first targeted because of these visible surface macroseeps. However, macroseeps reveal only part of the seepage spectrum story and fail to offer strong location clues since they escape along larger fault conduits. But hydrocarbon microseeps of light hydrocarbon gases are now being used to more accurately locate oil and gas reservoirs at depth. These microseep gases may be invisible to the human eye, but to gas chromatographs and natural microbial cultures they are clearly discernible and are very potent indicators of hidden reservoirs with chimney signatures.

Very small and very buoyant molecules of light hydrocarbon gases – methane, ethane, propane, butane and C5+ – naturally escape from oil and gas reservoirs and penetrate permeable reservoir seals. All reservoir seals, even shales and salts, can be penetrated by these tiny treasures. These C1-C5+ gases travel vertically through micro fractures up to the surface atmosphere forming a dynamic and measurable gas chimney signature. There are many seismic data sets that confirm such active gas chimneys. These direct indicator gases can be measured at the surface in both soil and offshore sediment samples.

The '3G' approach

Expensive geophysical data often determine where a prospect or development well is to be drilled. Such seismic data are imperative for making these structural interpretations. Unfortunately, disappointing dry holes tell us that not all seismic traps contain hydrocarbons. As an industry we still drill an unacceptable – and unnecessary – number of dry holes. We must improve our exploration accuracy and optimise our development programs. The combination and full integration of geology with geophysics and geochemistry ­­– the new 3G strategy ­­– is leading us to additional discoveries and more accurate wells.

Microseepage surveys are very precise tools for reconnaissance observations and for ranking seismic prospects. Detailed microseep signatures have been shown to reflect original reservoir heterogeneities before drilling and also accurately track dynamic reservoir pressures as production matures. Many more off-set and in-fill well locations must be executed in our maturing reservoir assets – drilling where we have already discovered reserves. Microseepage surveys are ideal to identify reservoir compartments and alternate zones for mature asset operators. Using such fast, low cost, and efficient microseepage methodologies allows operators to test for and squeeze out more reserves where 3D seismic is not practical nor affordable.

Stratigraphic detectives

As structures become more and more scarce, we must look ahead for more subtle stratigraphic traps as future reserve builders. MENA basins are very much under-explored from a stratigraphic trap point of view. Our extensive seismic database has previously focused on finding structures and thus leaving stratigraphic prospect interpretations under-utilised. Just as some explorationists are 'mining' old log library data for by-passed pay, old seismic data sets can be carefully re-examined for stratigraphic potential using microseepage surveys as lead indicators. Such 3G integration will breathe new life into mature provinces. Microseepage surveys will soon be the preferred exploration tool for discovering stratigraphic reservoirs.

Knowing natural seeps

Microseep surveys are very flexible in their design and scope. This translates to great efficiency and low cost. Microseep surveys commonly follow seismic programmes and rank their numerous structural prospects. These post-seismic surveys help focus limited drilling budgets and avoid wasteful dry holes. In MENA basins large-scale reconnaissance surveys – sampling expansive frontier areas prior to expensive seismic shoots – are also conducted. In mature producing areas, more detailed sampling patterns to locate by-passed reservoir compartments, offset well locations, and help plan waterflood and in-fill drilling projects are completed. The number of samples, sample densities, and sampling patterns are specifically matched to the exploration programme and expected targets. For reconnaissance surveys a single-vehicle crew can complete over 65 linear kilometres in one day. Sampling densities generally range from 100 metres (development drilling situations) to 500 metres (reconnaissance delineations). By design the collection methodology is very robust and extremely efficient. Men on foot with shovel/augers, and GPS create no environmental issues.

Little things, big results

For the last 25 years, Geo-Microbial Technologies (GMT) has progressed to become the premier microseepage provider. GMT conducts hydrocarbon microseep surveys throughout MENA and the rest of the world ­– both on and offshore. GMT collects shallow soil and sediment samples and then uses 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. Working with small and large independents, integrated majors, and NOCs, GMT has successfully integrated microseep data for selecting commercial discoveries (>70 percent) and predicting dry holes (>80 percent).

A MOST survey tests for specific microbes which consume thermogenic microseep gases. These natural 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, GMT supplements the microbial MOST 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. The combination of microbial and sorbed soil gas surveys is the strongest, most integrated and cost-effective microseepage exploration strategy available. Offshore analyses are completed on seafloor sediment samples (gravity or piston core samples).

Discover and recover

Besides hydrocarbons, oil and gas reservoirs are also home to vast microbial populations. These microbes do not reflect reservoirs as part of the microseepage chimney signatures. They live in the reservoir feeding on carbon sources and multiplying – always multiplying. In some respects, this causes a reservoir to act more as an immense bioreactor rather than as a simple 'container' for oil and gas. As production engineers realise, some microbial activity is good and some is very harmful to production practices. The huge expenditure for biocides for H2S and sulfide control is testament to the efficacy of microbial growth. So why not make microbes work for enhanced production rather than against it? GMT and its subsidiary, The LATA Group, provide for the manipulation of reservoir ecosystems and offer select formulae of nitrate-based treatments which will allow beneficial microorganisms to outcompete detrimental microorganisms. Beneficial microbial populations will control and prevent reservoir souring and corrosion, as well as create an in-situ IOR ecosystem including surfactant, CO2 and acid production.

The world of petroleum offers many enticing clues. As exploration detectives we have seen budgets stretched and tolerance for dry holes rejected. The adoption of low-cost microseepage surveys obviously increases drilling accuracy and efficiency. The ability to discover stratigraphic reservoirs and optimise mature fields boosts microseepage acceptance. 3G integration will expand advanced exploration and production projects.

Daniel Hitzman is the of President iGeo-Microbial Technologies, Inc. Raised in Oklahoma surrounded by oil and gas, Hitzman received his geology degree from Carleton College. Since 1985, he has supervised all GMT service operations and travels worldwide completing geochemical exploration surveys. He has authored 30 papers and presentations for the petroleum industry as an AAPG, SEG, and SPE member.