Posted by: silalahi | June 22, 2007

Sequence and Parasequence Concepts

How r u doin’ folks,

Here I wanna share about the confusing terminology between “sequence” and parasequence”. First when I read sequence stratigraphy concepts written by Posamentier, Summerhayes and Haq, I got the point that sequence is higher order sediment package than parasequence. Sequence (in Posamentier et al) is concluded as sedimentary package bounded by unconformity below and above or its correlative conformity seaward. Posamentier et al (1989) here was explaining that sequence is a sedimentary package which shows the conformable succession of a sedimentation. So, it was concluded that a sequence is a conformable series of a sediment deposited from lowest sea level condition to next lowest sea level condition in a basin.

Parasequence, what is  a parasequence?. Parasequence is a conformable succession of a sediment bounded by a deepening event of a such basin. parasequence often stacks and amalgamated in one relative sea level condition. For example, in condition of relative sea level slow rise/stillstands we could find a progradation of multiple parasequences, in condition of relative sea level rise we could find retrograded parasequences. Then, in a condition of  relative sea level keeps in balance with rate of sediment supply we could find agraded parasequences.

What is the application of these terminology (sequence and parasequence)?

If we can find the subaerial unconformity (sequence boundary, based on Posamentier et al ‘s concept) and traced that marker around a given basin, then we will be able to minimalize the probability of misscorrelation around the basin. If we talk about sequence it’s meant that we talk in basin scale. For sharping the resolution of correlation than we grouped those sediment interval in one sequence to some parasequences. If we’re talking in parasequence it’s mean we are talking in reservoir scale. Many reservoir zones nowadays in such basin operated by PSC Companies are divided in some parasequences. In the book written in 1989, Posamentier et al also make a differentiation between sequence and parasequence based on the sediment thickness and time comsuming for deposition.

In Catunenau (2003), sequence and parasequence is grouped in extremelly different model. Sequence (catuneanu, 2003) is a sedimentary package bounded by sequence boundary (it could subaerial unconformity in depositional sequence model or maximum flooding surface in genetic sequence model or maximum regression surface in trangressive-regressive sequence model), and parasequence in catuneanu is categorized as a litostartigraphy concepts.

Catunenau (2003) is trying to be objective and reduce the confusing of sequence stratigraphy terminology, but it  make sequence stratigraphy itself more confusing for the student.

Then if we read the paper written by Nummedal et al (1995) then we could reveal the misconception between parasequence and sequence. Sometimes I dont agree with the concepts of Posamnetier et al, cause the differentiation of sequence and parasequence based on the thickness range and time comsuming, but it doesnt mean that I completely dont agree with the POsamnetier’s concepts, it just about the thickness and time comsuming.

In Nummedal (1993), we could find that parasequence and sequence are extremelly different if viewed trough the sea level relative fluctuation. Nummedal et al (1993) concluded that a parasequence is a sedimentary package bounded by water deepening (final trangressive surface) and sequence is a sedimentary package bounded by water shallowing (regressive surface of erosion). Nummedal et al (1993) said that parasequence and sequence can have same time comsuming and same in thickness. So, the differentiation of sequence and parasequence based on time consuming and thickness cannot be widely used. For some case those parameters are applicable, but not really in whole basins.

Parasequence is deposited in a such condition of one full cycle of relative sea live rise without falling situation. So, the parasequence is formed from the highest relative sea level to the next highest relative sea level without falling limb. For that situation, a parasequence should have normal regressive sediement and tarangressive part sediment. Normal regressive sediemnt and trangressive strata is separated by initial trangressive surface.

Sequence (Nummedal, 1993) is deposited in such condition of one full cycle of relative lowest sea level to next lowest sea level. So the cycle in sequence is from low level – rise-then low level again. Parasequence has different scheme, that is high level – rise (but normaly regressive)-high level again.

An accurate model for packaging sedimentary rocks from basin scale to reservoir scale is the key succes for grabbing opportunities in developing and exploring oil/gas field.

Sorry if i didnt upload picture and animation for discussing this topic, it just was caused by I didnt have those things yet. I invite all my friends to discuss this topic while I am trying to edit this presentation to be more excellent.

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