Services

Correlation of deep marine Palaeogene sediments

Correlation of deep marine Palaeogene sediments (South Viking Graben), constrained by palynoevents.

Sequence Stratigraphy

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Since its conception in the mid-1970s, sequence stratigraphy has become an important technique in understanding basin chronostratigraphy. The recognition of sequences on seismic data leads to a better understanding of facies distribution which in turn leads to more accurate basin models, making the prediction of criteria required for hydrocarbon entrapment, such as reservoir, seal, source and migration pathway, more realistic and dependable. Fugro Robertson specialists have applied the principles of sequence stratigraphy to many sedimentary basins worldwide with the following objectives:

  • Identification of regional sequence boundaries from seismic data and well logs in conjunction with biostratigraphic calibrations
  • Integration of quantitative biodata to enable recognition of systems tracts and maximum flooding surfaces within sequences/parasequences
  • Definition of systems tracts from detailed seismic interpretation and well log analysis
  • Biostratigraphic definition of the age range of each sequence identified
  • Palaeo-water depth interpretation within each sequence, to provide detailed rock-based models of sediment distribution patterns within each given sequence
  • Identification of the interplay of eustasy, subsidence rate, uplift rate in source terrain and sediment supply to account for sediment 'accommodation rates' within the basin
  • Presentation of integrated play concepts which allow the identification of exploration play fairways, including type, geometry and spatial distribution.

While biostratigraphy plays a key role in this procedure, Fugro Robertson's facility to conduct detailed seismic interpretation and reservoir geology/sedimentology enables a fully integrated multidisciplinary approach to basin modelling.

StS™ - Stratigraphy to Seismic

The Stratigraphy to Seismic (StS™) technique has been developed by Fugro Robertson in order to providing stratigraphic interpretations in a digital format for integration with seismic data. These interpretations are derived from standard biostratigraphic analyses and are produced as a set of curves provided in .LAS format which can be plotted on seismic lines within workstation applications

Example of Stratigraphy to Seismic

Interpreted Geochronology Log - orange trace

Digital biostratigraphic data are compared to a stratigraphic standard, which contains key biostratigraphic events. These ages can be calibrated in millions of years (Ma) to various geochronologies. Any corresponding matches are produced as a cross-plot of age (Ma) and depth (feet/metres). This cross-plot is then used to define a line of interpreted geochronology which consists of an estimated interpretation of the age at any point in the well. Depositional rates and trend changes can be identified and onlapping sequences and truncations dated. These features on the curve can be readily matched against seismic reflectors and related to differences in seismic amplitude.

Discontinuity Log - red trace

The log is designed to show the nature and location of both the major breaks (regional unconformities and faults) as well as the subtle shifts caused by changes in the rate of deposition. In sections with little or no amplitude contrast in the seismic data, major breaks can be readily identified.

Reworking Index - green

The reworking index provides a representation of reworking based upon a comparison of the age and abundance of flora and fauna recovered in each sample with the age defined by the interpreted geochronology log.

Palaeobathymetry Log - blue trace

This log is an interpretation of the relative changes in palaeobathymetry and is scaled from non-marine/terrestrial to abyssal.

Wellsite Biostratigraphy

Papua New Guinea

A mountain-top rigsite in Papua New Guinea. This well was monitored by Fugro Robertson palynologists

Spain

Offshore Spain, the scene of a Fugro Robertson biomonitoring project.

Purpose

  • To monitor the stratigraphic progress of wells
  • To assist in setting casing points
  • To monitor coring programmes
  • To confirm age dependent TDs
  • To steer horizontal wells (biosteering)

Personnel

Wellsite operations may involve any of the biostratigraphic disciplines (micropalaeontology, nannopalaeontology or palynology) and in certain situations it may be necessary to run several disciplines together (usually micropalaeontology and nannopalaeontology, less commonly micropalaeontology and palynology).

Continuous 24-hour monitoring can be provided using 2 biostratigraphers, each working a 12-hour shift. There are also occasions when a fully qualified Fugro Robertson technician has to work alongside the specialists to prepare samples for analysis. Logistics

Standard ("picked residue") micropalaeontological and nannopalaeontological analyses involve no hazardous chemicals and require minimal equipment. These types of analyses are conducted in the logging unit. Wellsite palynology and thin-section micropalaeontology need a larger amount of equipment - in the case of the former, a mobile laboratory complete with fume cupboards needs to be set up, while for the latter it is necessary to despatch thin sectioning equipment. Due to time contraints, thin sectioning and palynological preparation are ideally performed by a Fugro Robertson technician, so freeing the biostratigrapher to concentrate on analyses and to keep up-to-date with the drilling status.

Wellsite experience

Fugro Robertson has carried out wellsite biostratigraphy in the following regions:

  • Barents Sea
  • Throughout the North Sea
  • Offshore Ireland, France, Spain and Portugal
  • Morocco, Côte d'Ivoire, Cameroon, Equatorial Guinea, Gabon and Namibia
  • Kenya, Somalia and Egypt
  • Oman, Qatar, UAE, Kuwait, Syria, Iran & Pakistan
  • Indonesia and Papua New Guinea
  • South America - Peru and The Falklands

Biosteering

Corrective Biosteering

An example of corrective biosteering of a horizontal well path in the Valdemar Field (offshore Denmark), utilising nannofossils ('N' zones are nannofossil assemblage zones) (after Fine et al.,1993).

The biosteering technique is now widely recognised as an integral part of modern horizontal drilling operations. Biosteering involves provision of real-time stratigraphic data at the wellsite. Used in conjunction with downhole logging data, it provides a high degree of control and enables correction, or 'steering' of the well trajectory to maintain an optimum level within the reservoir.

Pioneering development

The technique of biosteering was first developed by Fugro Robertson stratigraphers for the chalk fields of the Danish sector of the North Sea, and has subsequently been applied to horizontal/subhorizontal drillings throughout the North Sea (in both carbonate and clastic reservoirs) and the Middle East (carbonate reservoirs). All three biostratigraphic disciplines have proved valuable for biosteering in various reservoir facies.

Major benefits

  • Biosteering assists in the positioning of the well at the optimum reservoir level.
  • It also enables recognition of (and correction for) faulting and/or unanticipated changes in stratal dip.
  • It may enable re-interpretation of seismic sections along wellpath.

By using the technique in conjunction with logging while drilling (LWD), even greater refinement can be achieved. One significant advantage of wellsite biosteering is its ability to indicate the direction and size of throw on a fault, which is all important when correcting the well trajectory to re-enter the target zone.

Pilot studies

For maximum effectiveness, an initial, quantitative office-based pilot study must be undertaken in order to develop a high resolution local biostratigraphic scheme for a relatively short stratigraphic interval, including the target horizon. Such pilot studies are ideally conducted using closely spaced core material from nearby wells, but, given prior experience in a field, they may be conducted on a pilot-hole drilled across the target zone prior to pulling back and drilling the horizontal section. Several biostratigraphic disciplines may be employed in the pilot study, to determine which would be most useful at wellsite.

World records

Fugro Robertson has extensive experience of biosteering in the North Sea, Oman, Qatar, the UAE and offshore Iran. Our biosteering services have assisted in the drilling of several world record horizontal distances.