Seismic Stratigraphy EPS 444 Dr. Faisal A. Alqahtani 2011 email:falqahtani@kau.edu.sa 3

Data Types

What is seismic data? – theory from a geologists perspective… In seismic reflection surveys the travel times are measured of arrivals reflected from subsurface interfaces between media of different acoustic impedance. In areas where subsurface horizons are generally shallowly dipping velocity varies mainly as a function of depth due to the different physical properties of the individual rock layers…

What is seismic data? – from rocks to traces (I) Seismic sections can be made from 2D outcrops and greatly aid our understanding of the link between geology and geophysics. In particular we can understand how the acoustic properties of the rock vary with lithology and depositional environment and how these are represented on seismic reflection sections. The example to the left comes from some deep water channel deposits in the Ainsa Basin, Northern Spain. This study of seismic expression of lithologies and link to depositional environment is called seismic facies analysis (more later).

What is seismic data? – from rocks to traces (II) The image to the left compares the outcrop-derived seismic section shown in the previous slide to a real seismic example from offshore West Africa. Notice the similarity in the geometry between the outcrop and seismic examples, especially the low-angle dipping surfaces (outcrop) or reflections (seismic). These represent laterally migrating point bars within the inner meander loop of the channel. What are the pros and cons of each data type?

Volume of data to be interpreted = How big is it? A 10 km x 10 km (100 km2) 3D survey at 25 m line spacing contains 400 x 400 which equals 160,000 traces… …the majority of 3D surveys now have line spacing of 12.5 m resulting in 640,000 traces per 100 km2 dataset! Time samples are recorded at every 4 ms, so a single trace covering 4 s in time = 1000 time traces resulting in 6.4 x 108 samples for our 100 km2 3D seismic survey. For each sample either 8 bits or 16 bits of digital storage required to record the amplitude value of the sample. Volume of data to be interpreted = number of lines x number of crosslines x number of time samples x number of data types

Why do we need to interpret seismic data? – geological analysis To image the structure of the earth’s subsurface to better understand its temporal and spatial evolution.

Why do we need to interpret seismic data? – geological analysis To image the stratigraphy of the earth’s subsurface to better understand its temporal and spatial evolution.

Why do we need to interpret seismic data? – applied aspects To locate structural features that may contain exploitable hydrocarbon resources.

Why do we need to interpret seismic data? – applied aspects To locate stratigraphic features that may contain exploitable hydrocarbon resources

How do we interpret it? We need four basic things to interpret seismic data: Eyes Seismic section Pencils Imagination Geological background

2D Seismic Section

How do we view it? Understanding of the relationship between features observed in vertical and map-view (timeslice) seismic sections can be aided by using 3D visualisation packages. In the example to the right from the Northern North Sea a chair display is used to observe the relationship between the cross-sectional and map-view seismic expression of a channel.

Why a workstation? automatic management of the data digitally stores any position, travel time and amplitude of an interpretation in the database display seismic trace in different formats (e.g. wiggle display, variable amplitude colour displays) can exploit 3D volume of data - scan through data; visualize in 3D; plot well trajectories & logs; add in 2D regional seismic lines to database can flatten seismic data to a specific event allows automatic picking by using interactive volume tracking computation power of workstation-> ‘process’ interpretations; generate attribute maps & calculate attributes in volumes export/import data to other programs (e.g. mapping packages, reservoir modelling packages, well-interpretation packages)

How long does it take? …can take several months/years! Data loading Data preview, choosing displays Basic interpretation of horizons and faults Refinement of interpretation Creation of isopach maps, depth maps Attribute analysis for detailed stratigraphic or structural analysis Further detailed reservoir studies …can take several months/years!

Why 3D? – Applied aspects… Better (spatial) imaging of the subsurface New reserves Fewer dry & marginal exploration wells Optimization of number and placement of production wells Rejuvenation of old fields Appraisal/development planning cycles reduced Improved field management during lifetime due to “direct” imaging of fluids

3D seismic data – cost benefits from Brown, 2003

3D seismic data – cost benefits from Brown, 2003