Hydrocarbon Accumulations

Introduction and Commercial Application This section will firstly examine the conditions necessary for the existence of a hydrocarbon accumulation. Secondly, we will see which techniques are employed by the industry to locate oil and gas deposits. 

Several conditions need to be satisfied for the existence of a hydrocarbon accumulation, as indicated in Figure 2.1. The first of these is an area in which a suitable sequence of rocks has accumulated over geologic time, the sedimentary basin. Within that sequence there needs to be a high content of organic matter, the source rock. Through elevated temperatures and pressures these rocks must have reached maturation, the condition at which hydrocarbons are expelled from the source rock. 

To detect surface anomalies caused by hydrocarbon accumulations often very small amounts of petroleum compounds have leaked into the overlying strata and to the surface. On land, these compounds, mostly gases, may be detectable in soil samples. 

In Section 5.2.8 we shall look at pressure-depth relationships, and will see that the relationship is a linear function of the density of the fluid. Since water is the one fluid which is always associated with a petroleum reservoir, an understanding of what controls formation water density is required. Additionally, reservoir engineers need to know the fluid properties of the formation water to predict its expansion and movement, which can contribute significantly to the drive mechanism in a reservoir, especially if the volume of water surrounding the hydrocarbon accumulation is large. 

In a normal pressure regime the pressure in a hydrocarbon accumulation is determined by the pressure gradient of the overlying water (dP / dD), which ranges from 0.435 psi/ ft (10 kPa/m) for fresh water to around 0.5 psi/ft (11.5 kPa/m) for salt saturated brine. At any depth (D), the water pressure (PJ can be determined from the following equation, assuming that the pressure at the surface datum is 14.7 psia (1 bara) ... 

Finally, it is worth remembering the sequence of events which occur during hydrocarbon accumulation. Initially, the pores in the structure are filled with water. As oil migrates into the structure, it displaces water downwards, and starts with the larger pore throats where lower pressures are required to curve the oil-water interface sufficiently for oil to enter the pore throats. As the process of accumulation continues the pressure difference between the oil and water phases increases above the free water level because of the density difference between the two fluids. As this happens the narrower pore throats begin to fill with oil and the smallest pore throats are the last to be filled. 

Structural maps display the top (and sometimes the base) of the reservoir surface below the datum level. The depth values are always true vertical sub sea. One could say that the contours of structure maps provide a picture of the subsurface topography. They display the shape and extent of a hydrocarbon accumulation and indicate the dip and strike of the structure. The dip is defined as the angle of a plane with the horizontal, and Is perpendicular to the strike, which runs along the plane. 

The estimated probabilities of each of these events occurring are multiplied together to estimate the POS, since they must a//occur simultaneously if a hydrocarbon accumulation is to be formed. If the POS is estimated at say 30%, then the probability of failure must be 70%, and the expectation curve for an exploration prospect may look as shown in figure 6.9. 

Appraisal activity, if performed, is the step in the field life cycle between the discovery of a hydrocarbon accumulation and its development. The role of appraisal is to provide cost-effective information with which the subsequent decision can be made. Cost effective means that the value of the decision with the appraisal information is greater than the value of the decision without the information. If the appraisal activity does not add more value than its cost, then it is not worth doing. This can be represented by a simple flow diagram, in which the cost of appraisal is A, the profit (net present value) of the development with the appraisal information is (D2-A), and the profit of the development without the appraisal information is D1. 

Liquid hydrocarbons accumulated in non-condensible blowdown drums, originating from safety valves, closed drain headers, knockout drum drainage, etc. Facilities are normally provided at the drum for weathering volatile liquids and cooling hot liquids before disposal. 

Interpretation for irreducible water saturation assumes that the rock is water-wet or mixed-wet (water-wet during drainage but the pore surfaces contacted by oil becomes oil-wet upon imbibition). If a porous medium is water-wet and a nonwetting fluid displaces the water (drainage), then the non-wetting fluid will first occupy the larger pores and will enter the smaller pores only as the capillary pressure is increased. This process is similar to the accumulation of oil or gas in the pore space of a reservoir. Thus it is of interest to estimate the irreducible water saturation that is retained by capillarity after the hydrocarbon accumulates in an oil or gas reservoir. The FFI is an estimate of the amount of potential hydrocarbon in... 

Williams, U.P., J.W. Kiceniuk, and J.R. Botta. 1985. Polycyclic Aromatic Hydrocarbon Accumulation and Sensory Evaluation of Lobsters (Homarus americanus) Exposed to Diesel Oil at Arnold s Cove, Newfoundland. Canada Tech. Rep. Fish. Aquatic Sci. 1402. 13 pp. 

The relations shown in Figure 5.10 have a wide application to problems of fluid flow through permeable material. One of the most important applications for recovery of hydrocarbon is that there must be at least 5 to 10% saturation with the nonwetting fluid, and 20 to 40% saturation with the wetting fluid before flow occurs. Thus, for hydrocarbon (the nonwetting fluid), there must be a minimum of 5 to 10% saturation of the pore space before the fluid can move through the partially saturated or unsaturated formation and accumulate. Conversely, every hydrocarbon accumulation has a quantity of hydrocarbon that is not mobile since it is at or below a saturation of 5 to 10% and is thus not recoverable. 

In the present paper, emphasis will be placed on findings obtained in the last three years. Attention will focus on aromatic hydrocarbon accumulations and on the formation, retention, and structure of metabolites. Material appearing in recent reviews will not be emphasized (2,3,4). 

Evidence pertaining to the influence of compounds such as PCBs and heavy metals on aromatic hydrocarbon accumulations in marine organisms were discussed in recent review papers (3,4). 

Environmental temperature influences the degree of hydrocarbon accumulation in marine fish. Collier et al. (10) found that depressed temperature (4° vs. 10°C) resulted in significantly (P<0.05) increased retention of dietary C-naphthalene in brain, liver, kidney, and blood of coho salmon (0. kisutch). Thus, environmental conditions can have a pronounced effect on the disposition of hydrocarbons in exposed marine species. 

Invertebrates (crustacea and molluscs) readily accumulate hydrocarbons when exposed for more than a few hours through surrounding water. In crustacea, thoracic and abdominal sections (21,22), gills (22,23), stomach (22, 23), hepatopancreas (23), muscle (23), gonad (23), and blood (23j are sites of hydrocarbon accumulation. In molluscs, gills (24,25), adductor muscle (24, 25), viscera (25), mantle (24,25), and foot (25) are tissues in which hydrocarbons were identified in challenged organisms. 

Effluent woter from the Primory Woter/Condensate Separator is essentially hydrocorbon free and is discharged into the Drain Sump Coisson, which also collects the other effluent streams from the platform droinage systems. Any hydrocarbon accumulated in the droin caisson is recovered in the Skimmer Vessel, stored and finolly injected into the pipeline. 

As part of the over-all topic of the role of inorganic ions in the formation of economic accumulations of hydrocarbons—i.e., oil-fields— the analogy with the formation of ore bodies stands out clearly. The closeness of the analogy is revealed in a number of instances where hydrocarbon accumulations actually accompany the formation of mineral deposits. Such occurrences are hydrothermal in character (117), and while the oil deposits are trivial in amount, the association is very real, being most commonly observed in the case of ore deposits involving lead, zinc, and uranium. 

The eggs of B. germanica contain the same types of hydrocarbons as the hemolymph, HDLp, and cuticle of the adult female. Only 150 pg of hydrocarbons accumulate on the epicuticular surface whereas up to 450 pg accumulate within the female during the period of egg maturation (Fan et al., 2002). The internal hydrocarbons are divided primarily between the ovaries, fat body, and 150 pg of HDLp-bound hydrocarbons in the hemolymph. During oocyte maturation ovarian hydrocarbons increase by more than 65-fold - from 3.5 pg on day-1 to 232 pg on day-8 (Fan et al., 2002). However, after oviposition on day-9, ovarian hydrocarbons decline to only 8.2 pg, demonstrating that hydrocarbons were associated with the ovulated oocytes. Radiotracing results indicate that they serve as components of the cuticular lipids of the embryos and first instars (Fan and Schal, unpublished results). 

More deep bore holes should be drilled in carbonate sequences not directly associated with hydrocarbon accumulations to obtain diagenetic data and information on carbonate composition as a function of geologic age. 

Machel H.G. (1989) Relationships between sulfate reduction and oxidation of organic compounds to carbonate diagenesis, hydrocarbon accumulations, salt domes, and metal sulfide deposits. Carbonates and Evaporites 4,137-151. 

Eleven, thirteen and twenty alkylbenzenes were identified among the volatiles in the boiled crayfish tail meat, hepato-pancreas and the pasteurized crahmeat, respectively. The alkylbenzenes and the naphthalenes might have come from environmental pollutants. Lee et al. (30) reported rapid uptake of naphthalene in marine fish. Several chlorobenzenes identified in the crayfish and crafcmeat samples possibly were degradation products of various pesticides. Neff et al. (29) reported that aromatic hydrocarbons accumulated in fish to a greater extent and were retained longer than the alkanes. Phenol was also identified in all three samples. The medicinal odor of phenol contributed negatively to these products. KUbota et al. (28) identified xylenes and phenol as volatile components that contributed undesirable odors to cooked krill. 

Dwyer, L.A., Zamboni, A.C. and Blomquist, G. J. (1986). Hydrocarbon accumulation and lipid biosynthesis during larval development in the cabbage looper, Trichoplusia ni. Insect Biochem., 16,463 169. 

Wagrowski, D.M., Hites, R.A. (1996) Polycyclic aromatic hydrocarbon accumulation in urban, suburban, and rural vegetation. Environmental Science and Technology, 31(1) 279-282. 

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