STRATIGRAPHIC CONCEPTS

Oleh Unknown

Saturday 7 May 2016

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STRATA – TIME- STRATIGRAPHIC CLASSIFICATION AND PETROLEUM EXPLORATION

STRATA CONCEPT AND PETROLEUM EXPLORATION CONCEPT

•        Stratification controls:

–       Reservoir lateral extension.

–       Seal.

–       Trapping condition:

•        Structural Traps.

•        Stratigraphic Traps.

–       Petroleum system: from source rock to trap.

•        A in-depth understanding of stratigraphic concepts important of petroleum exploration concept.

CONCEPT OF STRATA

•        Strata, stratum, beds, layer

•        Stratification/ bedding/ layering

•        Strata as a basic concept in geology

•        Concept of Strata is important in understanding of reservoir rocks and seals

•        Concept of Strata is important in understanding correlation and time relationships

IDENTIFICATION OF STRATA AND STRATIFICATION

•        Vertical change in lithology

•        Vertical variations in lithologic characteristics such as : Grainsize, Color, Mineralogy,  etc

•        Some degree of lateral continuity

•        Vertical changes and variations in lithology varies from extremely thin, laminae, to very thick

BED AS BASIC LITHOLOGIC UNIT

•        Generally a uniform unit of lithology

•        Thickness varies from laminae to extremely thick

•        Vertical boundaries of strata are considered depositional interface

•        Lithology of a bed may change laterally but in an extremely gradual way

THE NATURE OF VERTICAL BOUNDARIES OF BEDS

•        GRADUAL OR TRANSITIONAL

•        ABRUPT OF SHARP

•        EROSIONAL

•        TRUNCATION

•        DISCONTINUITIES: gaps or breaks in deposition are quite common in stratal sequences:

CAUSES OF STRATIFICATION

•        Strata and stratification is caused by changes in the depositonal regime, mechanical energy, chemical of biological

•        Changes are caused by climate/weather, sealevel change or tectonics/sediment supply

•        Changes may take place daily, annually or even in millions of years

THE TIME SIGNIFICANCE OF STRATA

•        Steno’s Law of Superposition implies that stratal or depositional surfaces are isochronous time surfaces

•        A stratum or a packet of strata represents of time period of deposition or contemporanous deposition

•        Seismic reflection surfaces are stratal surfaces and being former bedding surfaces they are geologic time surfaces and are essentially  synchronous (Vail et al, 1977)

STENO’S PRINCIPLES (1669)

•        Superposition of Strata

•        Original Lateral Continuity of Strata

•        Original Horizontality of Strata

Collorary: Vertical Accumulation of Strata

LAW OF SUPERPOSITION (STENO, 1669)

Modified statement: 

Law of Superposition:

The youngest strata are at top in an undisturbed sequence (Anthony, 1955, p. 83

HORIZONTAL LATERAL CONTINUITY
Steno, 1669)

Modified statement:

Law of Original Continuity:

The original continuity of water-laid sedimentary strata is terminated only by pinching out against the basin of deposition, at the time of their deposition.

(Anthony, loc.op.cit, p.83)

STRATAL CONTINUITY AND CORRELATION

•        Steno’s Law of Stratal Continuity established that strata or bed can be traced laterally indefinitely(?)

•        William Smith’s Law established that a bed (ls, ss, sh etc) can be identified in different sections by its fossil content.. Correlation by fossil fauna

•        Correlation is essentially lateral tracing of beds

•        As bedding planes or stratal interface are isochronous, correlation is interpreted as establishing time relationships of strata

STRATA IDENTIFIED BY FOSSILS (SMITH, 1816)

Modified Statement:

 Strata can be distinguished by their characteristic fossils  (Woodford, 1935, op.cit, p.5-6; Schenk, 1940, p. 1754-1755)

LAW OF FAUNAL SUCCESSION
 (ABBE GIRAUD-SOULAVIE, 1777)

Statement of principle:

Fossil differs according to their geologic ages (stratigraphic position); fossils in lower formations are unlike those in higher beds. Fossil floras and faunas succeed one another in a definite and determinable order (Anthony, 1955, p.89)

LAW OF FAUNAL SUCCESSION

•        In a succession of strata  units can be identified by the specific assemblage of the fauna of its fossil content top.

•        The fauna of the fossil content resembles closer to the modern fauna as one goes toward the top.

•        A geologic time scale can be established based on the faunal succession.

LATERAL CONTINUITY VS LATERAL TERMINATION

•        Steno’s Law of Lateral Contintuity of strata: Strata continues laterally except at the edge of the depositional basin or being truncated

•        Sequence Stratigraphy Lateral Termination of Strata: All strata terminates laterally by lapping out towards the base as well towards the top of the sequence boundary.

LATERAL TERMINATION OF STRATA

•        Base-lapping

–       Down-lapping towards the open sea

–       On-lapping toward land

•        Top-lapping

–       Off-lapping

STRATAL CONTINUITY AND CORRELATION

•        Steno’s Law of Stratal Continuity established that strata or bed can be traced laterally indefinitely(?)

•        William Smith’s Law established that a bed (ls, ss, sh etc) can be identified in different sections by its fossil content.. Correlation by fossil fauna

•        Correlation is essentially lateral tracing of beds

•        As bedding planes or stratal interface are isochronous, correlation is interpreted as establishing time relationships of strata

Tag : Geologi