3-G Exploration: Discover the MOST with GMT Microseep Surveys

by Daniel Hitzman

Explorers for oil and gas often talk about the need for data integration. But do they really mean it? Combining geological and geophysical studies is just the beginning. Full and effective integration -- leading to greater accuracy and success -- includes geochemical evaluations, specifically hydrocarbon microseepage surveys.

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.

Exploration Formula: Seeps + Seismic = Success

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. 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.

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 3-D seismic is not practical nor affordable.

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 underexplored from a stratigraphic trap point of view. Our extensive seismic data base 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 3-G integration will breathe new life into mature provinces. Microseepage surveys will soon be the preferred exploration tool for discovering stratigraphic reservoirs.

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 1,000 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.

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%) and predicting dry holes (>80%). 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 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).

Daniel Hitzman

Geo-Microbial Technologies, Inc. (GMT)

www.gmtgeochem.com  

email: danielh@gmtgeochem.com