The distribution of Hydrocarbons (HCs) within a basin depends on a number of factors notably the type of source material and the physicochemical history of the basin. Factors responsible for the distribution of HCs can be divided into two parts; syn and post accumulation factors depending on when they come to play.
Syn-accumulation factors affecting the distribution of HCs include those that are inherent in the basin during its generation and accumulation. Such factors include organic matter (OM) type, and the physicochemical conditions of the basin.
ORGANIC MATTER (OM) TYPE
About the most fundamental criterion affecting the distribution of oil and gas within a basin is the source material. Tissot and Welts; 1982, in their classification of OM types recognised 3 types. (type I, II and III) based on the origin of richness. Types I and II are generally of marine origin derived from algal materials and considered to be rich sources of liquid HCs. On the other hand, type III OM has a continental source and is presumed to be a progenitor of petroleum gas. It has also been proven however that the exposure of both types I and II OM and their generated oil to high temperature will thermally degrade them to gas sources and gas respectively.
PHYSICOCHEMICAL HISTORY OF THE BASIN
The conditions included herein cover a wide range among which are the chemistry of the basin rocks, chemistry of subterranean fluids, structurally styles within the basin, hydrodynamic gradients, sedimentological peculiarities of the basin, temperature and pressure regimes.
The temperature and pressure regimes as well as the rock and aqueous fluid chemistry will affect the type of hydrocarbon pooled and its movement within the pore space. For example, it is common knowledge that oils from carbonate environments tend to be heavier and richer in S compounds than their clastic counterparts. The structural styles, sedimentological peculiarities and hydrodynamic gradients largely control the positions where the HCs are pooled. For example, syn-sedimentary structure like growth faults are known to act as conduits for HC migration. Depending on the timing of oil generation vis-a-vis the opening and transmissibility within the fault zone, it can act as a feeder for adjoining traps.
POST ACCUMULATION FACTORS
Though the concentration of HCs within a trap marks the final phase of oil and gas accumulation, pooled HC is often subjected to a range of physicochemical conditions under which they are potentially unstable. The instability is derived from a combination of the types of conditions within the reservoir and the complex nature of HC composition. While some of these changes may not senso-stricto affect the distribution of HCs within the basin, the go a long way in altering the nature of the HCs.
Some of the post accumulation factors considered include the following
As earlier indicated, both the HC source material and pooled HCs are subjected to thermal energy with increasing depth and rising temperature. Pooled oil become lighter by a disproportionation process which increases the amount of low molecular weight petroleum as the expense of the heavier components (Evans et al.; 1971). The process ultimately results in the formation of highlystructured aromatic type residue and hydrocarbon gas.
This involves the precipitation of asphalthanes from heavy to medium crude oils by the dissolution of large volume of gas or light HCs (> C6). The process occurs naturally but it has been successfully carried out in the laboratory and is now used on a routine basis to separate asphalthanes from other crude oil constituents. The products of de-asphalting and thermal degradation are similar except that the former is usually a localised phenomenon while the latter is often regional in nature. Also, bitumen components of de-asphalthaned oil exhibit lower maturity and relatively lower C-isotope ratios.
Fresh Water washing and Microbial Degradation
HCs pooled at relatively shallow depths where the subterranean fluid has some linkage with meteoric waters are susceptible to biodegradation and removal of water soluble compounds. Though both processes are mutually exclusive, they tend to occur together because they have a common causative factor. Biodegradation involves the selective digestion of certain types and structure of HCs by microorganisms carried in to the reservoir by meteoric water under aerobic conditions. Under anaerobic conditions within the reservoir, the microbes fix the oxygen from sulphates to oxidize the HC compounds. Microbial degradation of HC compounds seems to occur roughly in the following sequence
The effect of biodegradation on crude oils as seen in tar mats found in many parts of the world including south western Nigerian (Okitipupa area) and the high documented Atabaska [tar sands] sands of Canada.
Water washing involves the passage of aqueous fluid under-saturated with respect to HCs, through and oil bearing reservoir. This results in the removal of more soluble components. The net effect of water washing is that, a pooled oil gets heavier as in the case of biodegradation (fig 5-1).
Post Accumulation Tectonic Activity
This has the most notable effects of the redistribution of HCs within a basin. Tectonic activities are usually regional in scope and invariably lead to structural changes within the basin. Such structural changes could cause the destruction or failure of existing traps and the creation of new ones. The final resting place of the HCs thus depends on the location and competence of the new traps. Classic examples of this phenomenon are found in the gas fields of the southern North Sea, where gas ‘driven’ off the carboniferous coal measures by the Alpine orogeny got trapped in overlying Rotleidgens sandstone.
April 30, 2019
September 23, 2019