Two general procedures are relied upon to prevent ODP from drilling sediments that might contain hydrocarbon accumulation.
The first is the site selection and review process, in which proposed drill sites with geological factors conducive to hydrocarbon accumulation are either eliminated from the drilling program or relocated to avoid safety problems. Site review is performed by the JOIDES Pollution Prevention and Safety Panel and the Ocean Drilling Program Safety Panel. Some of the factors considered in the site selection and review process include the following:
The second procedure is the regular shipboard monitoring of the cores to insure that the sediments being drilled do not contain greater than expected amounts of hydrocarbons. The objective of hydrocarbon monitoring from a safety standpoint is to distinguish potentially hazardous accumulations of hydrocarbons from the background of the normal increase in hydrocarbon content with depth.
The prevailing guideline is that drilling should be stopped if hydrocarbons or hydrocarbon indications that suggest the presence of substantial accumulations of gas and oil are encountered (Ocean Drilling Program, 1992)
Identification of chemical composition and physical properties of any gas or solid petroleum substance is critically important for recognition of the presence or possibility of dangerous accumulations. The composition of gases may enable distinction between biogenic gas and thermogenic gas that has leaked upward from an underlying oil and gas accumulation. However, as indicated earlier, origin and composition are of secondary importance; it is the primary quantity of hydrocarbon gas and possibility of accumulation and trapping that pose potential danger.
The degree of generation of petroleum from source rock is primarily related to the temperature that the source rocks have experienced. Thermal maturity can be estimated from the chemical composition of sediment gases, distribution of solvent-extractable hydrocarbon constituents, pyrolysis assay, and color and reflectance of organic particles.
The temperature history or level of maturity must be known to evaluate the significance of hydrocarbon shows. For this purpose, it is necessary to (1) estimate the current capacity of organic matter to generate hydrocarbon and (2) compare the estimate with the actual presence of hydrocarbons, taking into account factors that could promote migration and accumulation of hydrocarbons.
The degree of organic matter maturation and its capacity to generate hydrocarbons may be estimated by an evaluation of (1) the content of C2-C5 hydrocarbons relative to methane (C1). The C1/C2 ratio changes from >10,000 for immature organic matter to <100 for mature organic matter; and (2) the Rock Eval pyrolysis parameters that provide evidence of the presence of migrated hydrocarbons.
Evidence for the presence of migrated hydrocarbons may be discerned by two main factors:
The hydrocarbon monitoring program consists of three main elements:
In addition, some measurement of organic carbon content is required. TOC can be obtained either from the Rock Eval TOC module or from the difference between the total carbon from elemental CHNS analysis and carbonate carbon measured by coulometry.