BLACK GOLD CHRONICLES 1: GEOLOGICAL DISTRIBUTION OF PETROLEUM

  Agbor Taku Junior    April 30, 2019    0

Introduction

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

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

  • Thermal alteration
  • De-asphalting
  • Fresh water washing
  • Microbial degradation
  • Hydrodynamic gradient and
  • Tectonic activity

Thermal Alteration

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.



De-asphalting

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

  • N-alkanes (< C-25), isoprenoid alkanes, low ring alkanes and aromatics.

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.







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