seismic interpretation

INTERPRETATION After acquisition and processing has been done the seismic data need to beinterpreted. Seismic interpretation is the process of determining information about thesubsurface of the earth from seismic data. Three seismic sections are manually colouredaccording to reflection terminations. The sequence boundaries such as unconformitysurfaces are marked. Then, these sections are interpreted in terms of depositionalenvironments and tectonics. EXERCISES: SEQUENCE & FACIES ANALYSIS Manually Interpreted Seismic Section-1 Seismic section consists of a super sequence comprising of many subsequences.

INTERPRETATION After acquisition and processing has been done the seismic data need to beinterpreted. Seismic interpretation is the process of determining information about thesubsurface of the earth from seismic data. Three seismic sections are manually colouredaccording to reflection terminations. The sequence boundaries

such as unconformitysurfaces are marked. Then, these sections are interpreted in terms of depositionalenvironments and tectonics. Sequence A:This sequence is made of a number of parasequences having same trend of deposition separated from each other by a number of unconformities. Sediments aredeposited under calm environment condition dipping in SE direction. Upper part of theparasequence shows onlap reflection pattern, which is lapout against underlying layer. Itis a transgressive sequence due to rise in sea level. Sequence-B: There are a number of progradational facies indicating deltaic depositions. These deltas are out-building basinward, indicating that the rate of sediment supply in the basin is more than the rate of subsidence during period of sea level fall. Sequence-B consists of six subsequences. Every sub sequence indicates differentgeometry of delta. Stretching of deltaic depositions mainly depends on velocity of theriver system, water depth and sediments supply. Subsequence-B1&B2, indicate decreasedvelocity of the river system, as indicated by oblique progradations.Seismically it ispossible to identify delta plain, delta front and prodelta. The gradient fall point of aprogradation is the subjacent shelf-edge (deltafront), rich of reservoir quality sands. Bothdeltaplain (landward part) and prodelta (basinward part) with reference to deltafront areexpected to contribute organic rich clays/shales. In sequence-B3, the elongated progradation indicates the velocity of the river system ason a gradually increased pattern. Whileas sequence-B4, denotes an eroded deltaplainfacies( erosional unconformity) indicated by toplap reflection pattern. However,Sequence-B5 and B-6, indicate as deposited in overall transgression without erosionaltoplap pattern andindicate a general delta out-build towards basin side. Sequence-C: This sequence shows a numbers of unconformities. Sediments are thickening in NW- direction and thinning towards SE-direction The SE side of this sequence is folded due to possible intrusions. Sequence D:This sequence shows parasequence sets of sediments depositions which are modified by compressional forces. Consequently, the middle part exhibits anticlinal structural configuration and SE part is deformed due to intrusion Sequence E:Sediment depositions are affected by post depositional tectonics thereby indicating anticlinal fold. It is interesting to note that the sediment thickness in the middleportion is more and toward both the sides is thinner in this anticlinal fold (clearly a caseof structural inversion). Sequence F: This sequence is made up of either salt dome or some magmatic intrusion.

Manually Interpreted Seismic Section-2 This section is consists of a number of sequences which are interpretations are as follow

Sequence E:Sediment depositions are affected by post depositional tectonics thereby indicating anticlinal fold. It is interesting to note that the sediment thickness in the middleportion is more and toward both the sides is thinner in this anticlinal fold (clearly a caseof structural inversion). Sequence F: This sequence is made up of either salt dome or some magmatic intrusion.

Sequence-A: In this sequence, sediment thickness is more which is indication of the moresupply of sediments from the river system and they are deposited along with subsidence.Unconformities present in this sequence indicate that the sediments deposited in differentperiods. A large number of faults can be seen which are developed due to structuraldeformation. Subsequence pattern shows that deposition takes place under high velocityof the river system. Sequence-B: It is consists of a number of subsequences separated from each other byunconformities. Upper layers are thin due to less supply of sediments from the riversystem while thickness of the lower layers increases with depth. This sequence has agentle slope going down to basin. Sequence-C: : A number of layers separated from each other by unconformities have variablethickness which may be due to subsidence and high velocity of the river system. Somelayers are folded due to structural deformation. Sequence-D: This Sequence may be basement rock which identified by the chaotic seismic reflection configuration.

This section is consists of a number of sequences which are interpretations are as follow: Sequence-A:The parallel trend of subsequence boundaries indicate that sediments are deposited under calm environment. They are onlapping against the upper eroded surface of sequence-B. The erosion of sediments is due to wave action. Sequence-B:This sequence consists of a number of sequences, each of which are separated by major unconformities dipping towards south direction .The thickness of thesediments increases towards south which shows that rate of accommodation space ismore than the rate of sediments supply. Sequence-C:This section shows deltaic configuration at the lower portion(?). Here sediment supply of the river system and velocity is more which causes elongated prodelta

Sequence-D:A homogeneous layered sequence of varying thickness, which was undergone structural deformation. There are many faults observed which are extendingvertically downward with some curved in nature. These faults are of both normal andreverse type. This structure is very similar

to salt dome and presence of constrained faultmaking it very prosperous for the accumulation and movement of the hydrocarbons. Sequence-E: This sequence indicates that they are made up of either salt dome or of magmatic intrusion