Page 156 - Geochemical Remote Sensing of The Sub-Surface

Geochemical Remote Sensing of the Subsurface
Edited by M. Hale
Handbook of Exploration Geochemistry, Vol. 7 (G.J.S. Govett, Editor)
9 Elsevier Science B.V. All rights reserved 133
Chapter 5

LIGHT HYDROCARBONS FOR PETROLEUM AND GAS PROSPECTING

V.T. JONES, M.D. MATTHEWS and D.M. RICHERS

INTRODUCTION

Surface geochemical prospecting for hydrocarbons includes a myriad of techniques
ranging from the direct detection of hydrocarbons escaping from subsurface
accumulations and source beds to identifying secondary responses in the soils, rocks and
biota in proximity to such accumulations or source beds. In the historical sense, the
observation of visible seepage of hydrocarbons is the oldest method of prospecting for
petroleum. Drake's historic well near Titusville, Pennsylvania, was drilled on the basis of
a seep in the adjacent creek bed. The relationship of such "macroseeps" to reservoirs was
well established by Link (1952), who stated: "A look at the exploration history of the
important oil areas of the world proves conclusively that oil and gas seeps gave the first
clues to most oil-producing regions. Many great oil fields are the direct result of seepage
drilling". In this respect, few would argue that the presence of a macroseep indicates the
presence of petroleum migration or surface source beds. Microseeps, or smaller scale
macroseeps, also occur because of the physical continuity necessarily imposed by nature.
These are invisible seeps, usually detectable only by sensitive instruments or by the
visible result of their effect on the near-surface environment. These microseeps, although
perhaps not as obvious or dramatic as macroseeps, are just as valid for the exploration of
undiscovered reserves. This chapter presents the conceptual and practical application of
microseepage detection and interpretation in the evaluation of areas for their subsurface
hydrocarbon potential.
Five factors are necessary to form a hydrocarbon reservoir. These are: (1) a source;
(2) a reservoir in which the hydrocarbons can collect or concentrate; (3) a means of
trapping these fluids in this reservoir (a seal); (4) a pathway to the reservoir (migration);
and (5) the proper timing such that the source, reservoir, seal and migration pathway are
present when required. Near-surface seepage of thermogenic hydrocarbons indicates the
subsurface presence of a mature source and migration pathway. It also suggests that, if
the hydrocarbons are reservoired, the seal is imperfect. This is true of both macroseepage
and microseepage. Patton and Manwaring (1984) found that even in an area of extensive
evaporites (Hugoton Field, Kansas), the seal was not perfect, and that microseepage
could be detected in the vicinity of the Syracuse Fault.