Page 31 - Drilling Technology in Nontechnical Language

22 Drilling Technology in Nontechnical Language Second Edition

Source Rock and Hydrocarbon Generation

Millions of years ago, plant and animal remains could sometimes
accumulate and become buried by material deposited above. Often this
was in slow-moving or stationary water environments like swamps, lakes,
coastal regions, and shallow seas. As these organic remains, together with
other nonorganic particles (clay minerals, fine sands, and silts) sank to the

bottom, thick beds of sediment built up over a long time (thousands to
millions of years).
As the organic-laden sediments became buried deeper, they were
subjected to increasing temperatures and pressures. The sediments, under
diagenesis, became sedimentary rocks. About 99% of all hydrocarbon
deposits are found in sedimentary rocks. Within the tiny pore spaces of
the rock, the organic matter also underwent chemical transformation.

Under certain conditions, oil is generated from organic remains

(fig. 2–1). The most important factor is temperature. Oil generation starts
at 50°C. Conversion to oil peaks at 90°C and stops at 175°C. This range of
temperature, 50°C to 175°C, is known as the oil window.

Fig. 2–1. Oil generation and migration

Below and above the oil window, decay of the organic remains will
generate gas. Below 50°C, biogenic gas (generated by microbes) or swamp
gas will result. Above 175°C, thermal gas will result. Temperatures at the
lower end of the oil window will generate heavy oils, with increasing

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