Background
Many types of geochemical methods have become useful and successful tools for petroleum exploration. Surface geochemical techniques, in particular, have been used with success since the 1930s. The science of geochemistry, in general, has matured considerably since the 1930s and especially in the 1980s. This has been the result of better analytical equipment and techniques, a long history of successful application of exploration (surface) geochemistry in the mining industry, development of sophisticated statistical techniques, and a greater understanding of soil and subsurface conditions and processes.
A wide variety of techniques in petroleum exploration have been grouped into a general category called unconventional methods; surface geochemistry is classified as one of them. Surface geochemistry is composed of several techniques that detect chemical and biological changes caused by hydrocarbon leakage from the reservoirs to the surface, particularly in the near-surface soil, atmosphere or in the ocean environment.
One of the first questions is: Will leakage of a reservoir occur if the overlying seal rock is assumed to be impermeable? In recent years, the answer that has come through observation and empirical data indicates that no rock type is completely impermeable and that migration of minute amounts of solids, gases, and liquids through the seal does occur. The universally impermeable shale has been found to be, under certain conditions, a highly fractured and productive reservoir. Environmental geochemistry has shown that hazardous liquids (such as petroleum) migrate effectively through clays, shales, and evaporates. Therefore, the surface geochemical anomaly is the end result of hydrocarbon leakage from a reservoir at depth. Nowadays, surface geochemical studies are considered as one of the most powerful and reliable methods for petroleum exploration with a high success rate (up to 98%).
Applications
Surface geochemical methods plays an important role with high efficiency in petroleum exploration and development as follow:
Petroleum exploration
- Recognition of active petroleum system in frontier basins
- Identification of structural and stratigraphic charged traps
- Leading seismic survey
Field development
- Early delineation of field limits
- Identification of by-passed pay or undrained reservoir compartments
- Evaluation of in-fill or step-out drilling locations
- Documenting hydrocarbon drainage over time
- Contribution to reservoir characterization studies
- Monitoring water-flood and CO2-flood operations for EOR
Analytical methods
There are generally two categories of surface geochemical methods: direct and indirect. The direct method is the detection and measurement of hydrocarbons (soil-gases) that are expelled from the subsurface into the soil substrate, but the indirect methods encompass all other forms of geochemical phenomena whose cause and presence are a by-product of soil-gas reactions with the soil substrate and atmosphere. The most important direct and indirect surface geochemical analyses provided by the Energy Researchers Ariana (ERA) Company are listed below.
Direct methods
- Soil gas
- Acid extraction
- Deabsorption
- Fluorescence
- Free-air
- Head-space
- Sniffer survey
Indirect methods
- Microbial (MPOG)
- Gamma-ray spectroscopy
- Iodine
- Helium
- Radon
- Trace elements
- Geobotany
- pH/Eh
- Remote sensing (RS)
Benefits
- Simple and environmentally friendly sampling
- Lacking halo effects – unlike the other surface prospection methods
- Reflection of seepage over time rather than only present-day seepage by sorbed HCs data
- The disappearance and reappearance of microbial anomalies in response to reservoir depletion and
- epressuring
- Not affecting by external disturbance factors
- Not affecting by fractures, overlying salt or other geological structures
- Reliable results are obtained even for structures having a complex geological formation
- A cost-efficient method in comparison with others
- Establishing a clear distinction between oil reservoirs, gas reservoirs and oil-bearing structures with a gas cap
- Applicable in both existing onshore (permanently frozen, continental and desert) and offshore sites
- High exploration success rate (up to 98%)