Contents of
April, 1998 - Vol. XIX No.2
SEISMIC VELOCITY ANALYSIS
FOR MATURITY ASSESSMENT: UPPER ASSAM BASIN, INDIA
R.K Mallick*, S.V. Raju* and K.D. Gogoi**
*Oil India Limited, Assam 786 602 India
**Department of petroleum technology, Dibrugarh University, Assam, India
Abstract
The Upper Assam foreland basin is
an important onshore petroliferous region of India with reservoir rocks ranging
in age from Paleocene to Miocene. Significant source rock intervals are found
in the Upper Paleocene-Lower Eocene Sylhet Formation and Upper Eocene-Oligocene
Barail Formation. In a previous study, the thermal maturity of source rocks
obtained from measurement of vitrinite reflectance were compared with sonic log
derived maturity. The relationship can be expressed mathematically, in simple
terms. In The present study the method has been extended to thermal maturity
evaluation from seismic velocity analysis. A good correlation has been obtained
between maturity from seismic data and measured/calculated vitrinite
reflectance values. This has enabled construction of thermal maturity map for
the entire study area. The use of seismic velocity analysis is simple, reliable
and does not entail any additional expenditure. It also has the advantage of
providing preliminary estimates of source rock maturity in undrilled areas of
the basin.
Back to Contents of this issue
SEISMIC VELOCITITIES
FOR EFFECTIVE PLANNING OF EXPLORATORY WELL IN VIRGIN AREA - A CASE STUDY FROM
NORTH BANK AREAS OF UPPER ASSAM
C. S. Singh, A. K. Khanna, S. N. Singh
GDAP Department, Oil India Limited, Duliajan,
Assam - 786 602
Abstract
The continuing exploration venture
of Oil India Limited (OIL) in the south bank of river Brahmaputra provided
possible clue of hydrocarbon accumulation in north-bank of the river. In
continuance to its extensive exploration programme, OIL acquired the PEL of
about 4200 sq. km covering the north bank of river and subsequently collected
about 1700 GLKM of 2D seismic data of different vintages from the erstwhile
ONGC to assess the various exploration objectives in the area.
Based on the interpretation of
these seismic data and other available geological information, OIL planned to
drill a few exploratory wells in the area. Simultaneous to the above
geoscientific work and planning for drilling, OIL deployed one of its 2D
seismic crew to acquire data in the area during the year 1995-96. It was felt
prudent to process these data on a high priority basis and review the
exploration target prior to spudding the first well in the area.
The acquired data processed using
Oil's in-house Landmark Seismic Data Processing system with Promax 6.1
processing software and the results were made available as expected. This paper
discusses the result of this processing with specific emphasis on the advantage
of interactive velocity analysis which provided reliable lead in guiding the
well planning. It has been observed that the seismic velocity macro model
estimated from the available drilling information in adjacent south bank wells
and the geological correlation from the south bank area, needed considerable
validation to meet the objectives. The generated seismic sections were
converted to depth section using the products of interactive velocity
analysis/manipulation and was a major input to the depth model.
Back to Contents of this issue
PROCESSING AND INTERPRETATION
OF GROUND MAGNETIC DATA ACQUIRED ALONG SILCHAR-IMPHAL-MOREH CORRIDOR (EASTERN
CACHAR AND MANIPUR AREA)
D. Rai and V. Ramaswamy
Geophysics Division, KDMIPE, ONGC, Dehra Dun -
248195.
Abstract
Magnetic data of Silchar-Imphal-Ukhrul
and Palel-Moreh corridors (eastern Cachar and Manipur) have been processed and
interpreted to bring out the basement configuration. The study reveals:
·
thickness
of sedimentary cover near Lakhipur (18 km east of Silchar) is about 6.5 km and
it gradually becomes thinner towards Imphal,
·
in the
Manipur valley the maximum sedimentary thickness is about 4.0 km and
·
in the
eastern Manipur Hills the sedimentary thickness again increases and attains a
value of about 6 km near Moreh at Indo-Burmese border where the ophiolite is
exposed at the surface.
The basement configuration along Silchar-Imphal-Moreh corridor
is arc type with convexity upwards. In brief, the study presents some new
results on the subsurface geology which may be used in hydrocarbon exploration
in the area.
APPLICATIONS OF MISFIT
FUNCTION TOPOGRAPHY IN GEOPHYSICAL MODEL OPTIMIZATION
Saurabh K. Verma*, Markku Pirttijarvi** and Sven-Erik Hjelt**
*National Geophysical Research Institute,
Hyderabad, India
** Department of Geosciences and Astronomy, University of Oulu, Oulu, Finland
Abstract
To proceed with any optimization
technique, a measure of the distance between the observation and the response
calculated from the model is required to be defined. This measure is called
'Observation function' or 'Misfit function' (MF, used in this paper). Several
norms (l1, l2, …., lp, etc.) can be used to define this function. The behaviour
of MF over a space described by a combination of model parameters can be
displayed in the form of a MF topography (MFT). Construction of the MFT
requires a thorough scanning of a vast model space.
The MFT maps can provide valuable
clues on the correlation between the parameters defining the model space. For a
dynamic (time-variant) system, such as a transient electromagnetic (TEM)
method, the MFT is found to vary as a function of time. Thus it is possible to
select suitable time-windows of observation which provide optimal resolution of
the model parameters. Since various TEM systems employ different types of
exciting pulses (sinusoidal, ramp, triangular, etc.) and recording time
channels, the MFT's can also be exploited in comparing the performance of
various TEM systems. Also, since MFT's are based on the scanning of a vast
model space, in essence they represent a process similar to the grid search
technique of optimization commonly used in finding non-linear parameters from
geophysical data.
The above applications of the MFT
are shown considering synthetic responses of i) a perfectly conducting
half-plane (minimal) model in frequently domain and ii) a conducting finite
plate model to commonly employed generic TEM systems. Both these models are
assumed to be immersed in a non-conducting medium for the sake of simplicity. A
field example from the nickel sulphide deposit, Mt. Keith South, Western
Australia is also studied to demonstrate how the MFT maps can be used to obtain
the model parameters.
Back to Contents of this issue
AN INTEGRATED CRUSTAL
MODEL ALONG NAGAUR-JHALAWAR GEOTRANSECT
D. C. Mishra, Bijendra Singh, V. M. Tiwari, S. B. Gupta, N.
Kameswara Rao and M. B. S. Vyagreswar Rao
*National Geophysical Research Institute,
Hyderabad, India
Abstract
An integrated crustal density
model along Nagaur - Jhalawar geotransect across the Aravalli fold belt is
constructed based on the modelling of gravity data using the constraints from
deep seismic reflection profiling results and near-surface geology. The
observed gravity field along the transect shows "high" Bouger and
Free-air anomalies over the fold belt and "lows" on its flanks over
the Marwar and Vindhyan basins. Results of 2½ dimensional gravity modelling
indicate that the gravity high in the central part of the profile is partly due
to a prismatic shaped high density body (3.09 g/cm3) in the lower crust
extending from 18 Km upto 45 Km and partly due to the exposed high density
metasediments of Delhi and Aravalli fold belt. The lows on the flanks have been
attributed mainly to the presence of low density granites and sediments. The
resultant crustal density model shows high density basement and lower crust
underlying the adjoining Marwar basin in the west and Vindhyan basin on the
east of fold belt. The large-scale thrusting of rocks along shear and fault
zones from east to west might have resulted from continental collision during
Proterozoic period.
Back to Contents of this issue
STUDY OF UTTARKASHI
EARTHQUAKE IN TERMS OF RUPTURE MODEL AND ISOSEISMALS
A. Joshi
Lecturer, Department of Geophysics,
Kurukshetra University, India
Abstract
Peak ground acceleration for
Uttarkashi earthquake has been compiled by modelling rupture process. Field and
simulated peak acceleration data and the isoseismal map prepared from synthetic
and field data have been compared.
Modelling of the rupture plane is
based on semi empirical method of Irikura (1986) which has been modified by
Midorikawa (1993). Modelling of the rupture plane by this technique gives peak
ground acceleration at the observation point.
After assuming the modelling
parameters of rupture plane, peak acceleration using this approach was
calculated at thirteen different stations that had recorded strong motion data
of Uttarkashi earthquake of 20th Oct, 1991. A comparison of simulated and field
peak acceleration less than 25% at six stations and less than 55% at seven
other stations, thereby confirming the parameters of selected model and
efficacy of the approach.
Hundred and sixty four different
locations surrounding the rupture model for Uttarkashi earthquake were taken
for simulation of peak ground acceleration. Peak acceleration at each location
was converted into intensity on MMI scale by empirical relation between peak
acceleration and maximum intensity on MMI. The comparison between the
isoseismal maps based on synthetic and field data shows that the elongated axis
of isoseismal map is dependent on the position of rupture plane and direction
of rupture propagation from nuclear point.
Back to Contents of this issue