A new formula for success

Searching for oil and gas deep underground is more and more like our current TV detective shows: 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 nails the bad guy. So it is with petroleum exploration and development today – sometimes the very smallest molecules are the key to guarantee success.

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 North American 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 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 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 and 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 optimize 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. North American 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-utilized. 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 breath new life into mature provinces. Microseepage surveys may turn out to 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 programs and rank their numerous structural prospects. These post-seismic surveys help focus limited drilling budgets and avoid wasteful dry holes. In North American 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 program and expected targets. Reconnaissance surveys are often simply conducted in public-right-of-ways along county or section roads. A two-man crew can complete up to 40 linear miles in one day with this strategy. The same two-man crew can cover at least one-and-a-half square miles with a gridded sampling pattern. Sampling densities generally range from 300 feet (development drilling situations) to 1000 feet (reconnaissance delineations). By design the collection methodology is very robust and extremely efficient. Men on foot with a shovel, small backpack and GPS create very little environmental or surface owner issues.

Little things, big results

For the last 25 years, Geo-Microbial Technologies (GMT) has progressed to become the leading microseepage provider in North America. GMT conducts hydrocarbon microseep surveys throughout North America 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 stripper well operators, both 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 that 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 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 escape from reservoirs as part of the microseepage chimney signatures. They live in the reservoir feeding on carbon sources and constantly 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 realize, 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 optimize mature fields boosts microseepage acceptance. 3G integration will exploit advanced exploration and production projects.

For more information Geo-Microbial Technologies, Inc. (GMT) can be found at www.gmtgeochem.com and www.latagroup.come or via email at danielh@gmtgeochem.com

BIO

Raised in Oklahoma surrounded by oil and gas, Daniel C. 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.