Chapter 3 - Geologic Time and Stratigraphy

Abstract

Abstract The concept of long periods of time being required for reservoirs to assume their present form is difficult to grasp, particularly for those individuals who track daily oil and gas production from reservoirs. However, the lengthy formative processes for hydrocarbon reservoirs can be understood, and this understanding is important for proper knowledge of why a reservoir is configured the way it is. The geologic time scale is divided into a series of time intervals that are based on significant events in the geologic record. Various temporal names applied to rock units commonly are used and must be recognized by people studying reservoirs. For a simple example, a Cretaceous reservoir rock was not deposited at the same time as a Devonian reservoir rock. The time during which a rock formed is dated by two means: absolute dating and relative dating. Absolute dating refers to the analysis of radioactive components in a mineral (within a rock), which provides the age at which the mineral formed (solidified) in the rock. Such techniques are used mainly for igneous rocks that cool directly from magma, but some chemically precipitated minerals and cements in sedimentary rocks can be dated in this manner. More common to the study of sedimentary rocks is relative age dating, where the age of a particular rock is determined relative to its position within a stratigraphic succession. If sedimentary rocks are crosscut by datable igneous rocks, sometimes the absolute age range of deposition of the sedimentary rock can be determined. An analysis of microorganisms in sediments and sedimentary rocks can provide a useful means of establishing rock zonations (biozones) and sometimes for determining absolute age. Micropaleontology, biostratigraphy, and palynology are critical disciplines in the petroleum industry, for exploration and for reservoir characterization. In addition to providing a means for absolute dating of sedimentary rocks, high-resolution biostratigraphy and palynology can aid in (1) interpreting stratigraphic intervals and their ages on seismic reflection profiles, (2) correlating between-well stratigraphic and temporal relationships, (3) determining sedimentation rates, and (4) determining depositional environments and changes in environments over time. Walther’s law of succession of sedimentary facies is key to understanding the origin of sedimentary deposits and reservoirs. It is a fundamental principle that is the backbone of stratigraphy. Stratigraphic sequences, such as those that comprise reservoirs, exhibit systematic and somewhat predictable vertical stacking patterns that are explained by Walther’s law. By understanding the vertical stratigraphy of a reservoir, one can make improved interpretations of the lateral (dis)continuity of reservoir intervals.