|SEISMIC - GEOCHEMICAL
SURFACE AND MARINE SURVEYS
INTRODUCTION TO GEOCHEMICAL SOIL-GAS PROSPECTING
Present day exploration for oil and gas requires a coordinated effort based on the successful integration of geophysics, geology, and geochemistry. Surface geochemical prospecting offers the explorationist a portable, cost effective tool to reduce exploration risk. Specifically, the analysis of light hydrocarbons in soil vapor, seismic shothole, and marine piston core samples are used to:
* quickly evaluate the
productive potential of unexplored regions;
"A look at the exploration history of the important oil areas of the world proves conclusively that oil and gas seeps gave the first clues to most oil producing regions. Many great oil fields are the direct result of seepage drilling."
Few would argue that the presence of a "macroseep" indicates the presence of petroleum migration or surface source beds. Microseeps, or smaller scale macroseeps, also occur and are usually detectable only by sensitive instruments. These microseeps, although perhaps not as obvious or dramatic as macroseeps, are just as valid for the exploration of undiscovered reserves.
Exploration Technologies, Inc. has developed collection techniques and instruments capable of obtaining and analyzing this hydrocarbon signal. The stratigraphic and structural mapping of geological and seismic data, coupled with the measurement of microseeps in the shallow subsurface, provide powerful means of evaluating and ranking prospective trends and traps.
The fundamental assumption of near-surface, geochemical exploration is that thermogenic hydrocarbons, generated and trapped at depth, leak in varying quantities towards the earth's surface in detectable amounts. The close association of near-surface geochemical anomalies to faults and fractures is well known. These fractures act as preferential pathways, focusing the flow of hydrocarbons from the source beds to the reservoir, and from there on towards the surface. Anomalous hydrocarbon concentrations are always real seeps, since active flux is necessary to overcome near surface interfering effects.
The most useful detection technique involves the measurement of the light hydrocarbons: methane, ethane, propane, iso-butane and n-butane. Because of their volatility, these light hydrocarbons are generally found in the free pore space, either in the vaporous or dissolved (in water) state. Worldwide survey results in marine and onshore environments have shown that the ratios of these light hydrocarbon components correlate with source maturity and the type of production (oil, gas, or condensate) in a region. Consequently, based on compositions, the technique can be used as a source rock tool applied at the surface.
Near-surface soil gas geochemical investigations require the analysis of free soil gases collected through a probe that is inserted into the ground. Probe sampling can be adapted for use in a variety of geologic terrains. The mobility of the soil gas probe sampling opens up large areas to geochemical exploration that are otherwise difficult to sample. Probe sampling is particularly worthy because of the low sampling cost and ease of access in rugged, roadless areas. With this method, small crews of only one or two persons can obtain large numbers of samples at minimal expense. Public and private landowners are agreeable to probe sampling because there is no surface damage from sample collection. This technique has been used successfully all over the world.
ETI has extensive experience in applying surface geochemical prospecting techniques to seismic shothole surveys. Because of the cost of permitting, surveying and drilling of shotholes, we strongly recommend the measurement of light hydrocarbons in seismic shotholes as a way of increasing the exploration information available along seismic lines. In addition, the collection and analysis of deeper samples (from shotholes) improves the quality and information content of the geochemical data. Hydrocarbon gases (C1-C4) found in bedrock may be efficiently sampled by analysis of the drilling fluids used to penetrate the rock. Best results are obtained when seismic shotholes are drilled with water (as opposed to air). Canned samples are taken of the drilling fluid, rather than drill cuttings, prior to loading the holes with explosive.
The hydrocarbon signal of exploration interest is also present in marine sediments. Samples in marine geochemical surveys are usually obtained from shallow coring devices, but hydrocarbons can also be measured directly in seawater using sniffer technology.
After collection, all samples are sealed, shipped to ETI's Houston laboratory, and analyzed for methane, ethane, propane, iso and normal butane, ethylene and propylene by flame ionization detector (FID) gas chromatography. Results are calculated and reported in parts per million by volume.
This established geochemical prospecting techniques can be applied to
aid exploration programs in any basin. Exploration Technologies, Inc.
offers state-of-the-art geochemical exploration technology and is the
most experienced surface geochemical service company in the world.§