NORTHERN CARNARVON BASIN
RESEARCH STUDY

A new study designed to investigate the timing, magnitude and extent of paleo-thermal effects, and to provide an independent assessment of maturity levels, using AFTA^®

Analysis of ~30 new AFTA^® (apatite fission track analysis) samples from 13-15 wells will be the basis for evaluating variation in thermal, tectonic and maturation histories throughout the Carnarvon Basin. Using the latest AFTA^® and THR™ (thermal history reconstruction) technologies, this regional study (the first of its kind for this area), will delineate the timing and extent of paleo-thermal effects in each study well, plus provide an independent test of maturity levels in Mesozoic source rocks. Integration of the new AFTA results with existing VR and FAMM data will arm explorationists with an independently-generated thermal and maturation history framework, which can later be used to constrain more detailed basin models, and thus help identify which play-types are best targeted in different parts of the Carnarvon Basin.

 

A proposal to industry

December 1999

 

Introduction:

This proposal to industry outlines the aims and objectives of a research study which will be undertaken in the northern Carnarvon Basin by Geotrack International Pty Ltd. Attachment A provides some background detail about the techniques that will be applied in this study. More details can be obtained by contacting Paul Green (61 3 7065 6463). Attachment B contains the application form, which should be returned by committed parties, prior to the 31st January 2000.

Geological Background:

Proven oil and gas fields throughout the Barrow and Dampier Sub-Basins of the greater Carnarvon Basin are based on numerous and diverse play-types, located in a complex structural setting. Principle reservoir rocks vary in age from Triassic in the Gorgon Gas Field, Late Jurassic in the Mutineer Oil Field, and Early Cretaceous in the Barrow Oil Field. The challenge presented to explorationists by such a broad range of possible drilling targets is further complicated by widely recognised suppression of Ro data. This problem mainly affects measurements in Jurassic and Cretaceous sediments, and stems largely from difficulties in identifying vitrinite in fine grain marine sediments. Uncertainties surrounding maturity levels currently make it difficult to accurately assess magnitudes of maximum paleotemperatures and burial depths in the Carnarvon Basin.

The tectonic evolution of the Carnarvon Basin is also complex, and has produced a broad array of structural traps, for which the "timing" of emplacement is a crucial element needing to be defined in each petroleum system. Both sub-basins are bound to the north-west by the Rankin Trend (a series of en-echelon horsts and grabens), and to the south-east by the Peedamullah and Lambert Shelves. Within the sub-basins, anticlinal trends occur at Barrow Island, the Mermaid nose, the Madeleine and Legendre Trends, and the De Gray nose. All of these structures are associated with unconformities of varying age and extent (both temporal and spatial), and so the timing of uplift and erosion, and amounts of section removed need to be accurately assessed for entry into constrained basin models.

Aims and objectives:

• Application of Apatite Fission Track Analysis (AFTA^®) to delineate the timing and extent of paleo-thermal effects in 13-15 wells (see figure above and provisional well list below).

• Comparison of AFTA-derived maximum paleotemperatures with VR (vitrinite reflectance) and FAMM (fluorescence alteration of multiple macerals) data, to independently assess true maturation levels of Triassic to Cretaceous units throughout the study area.

• Provision of constrained thermal history reconstructions for each study well, plus an interpreted overview of thermal history styles throughout the northern Carnarvon Basin.

• Comments on the variation of maturation and burial/uplift histories within the Carnarvon Basin.

Detailed work program:

• Process 2-3 AFTA^® samples from each of 13-15 wells. Sampling will concentrate on pre-Triassic to Cretaceous aged sediments in: De Grey-1, Goodwyn-1, Wandoo-1, Arabella-1, Candace-1, Barrow-1, North Gorgon-1, Vlaming Head-1, Fennel-1, Echo Bluff-1, Onslow-1, Loggerhead-1, West Muiron-3, Resolution-1 and Cape Range-1. (This list is provisional and changes may be made in response to suggestions from subscribing companies. Geotrack would also welcome company contributions of VR and FAMM data to this study.)
  
• Derive "timing of onset of cooling" and "maximum paleotemperature" constraints for each AFTA sample using the latest in-house Geotrack software, which incorporates measured Cl contents of each apatite grain analysed, as well as fission track age and length data. Where multiple thermal events have affected well sites, obtain subsequent "peak paleotemperatures" from the AFTA data.
  
• Convert VR data to estimates of maximum paleotemperature. Compare these with the AFTA results to assess the degree and extent of suppression affecting the VR data in each well.
  
• Construct maximum paleotemperature profiles for each well, using AFTA and selected-reliable VR results, and hence comment on possible mechanisms of heating and cooling in each well. Provided that paleotemperature profiles are linear, provide formal estimates of paleogeothermal gradients and removed section (with associated 95% confidence limits).
  
• Reconstruct thermal, burial and maturation histories for each of the study wells.
  
• Provide a synthesis of results, and an overview of the paleo-thermal events recognised throughout the study area based on the AFTA results and selected-reliable VR.

Study Deliverables:

Cost: Subscription to the study is $12,000 per company.

Price includes a full report containing the thermal history interpretations of each AFTA^® sample, THR^™ (thermal history reconstruction) for each well, paleogeothermal gradient and section removed analysis (where applicable), plus full presentation of the AFTA, VR and geological data.

Study commitment date: Notification that your company will be subscribing to this Research Study must be received by Geotrack on, or before the 31st January 2000.

(Please use the attached application form - Attachment B.)

Delivery date: May 2000

Confidentiality: The report will be available on a non-exclusive basis. Results will be held confidentially by Geotrack International Pty Ltd for 2 years after the completion of the study.

Attachment A: Technical background

Apatite Fission Track Analysis (AFTA^®):

AFTA is based on analysis of radiation damage trails ("fission tracks") within the crystal lattice of detrital apatite grains, which are a common constituent of most sandstones. The continuous production of new fission tracks through time, coupled to the reduction in track length as a function of temperature and time, provides the basis of the technique. Thermal history information is extracted from the AFTA data by modelling measured AFTA parameters through a variety of possible thermal history scenarios, varying the magnitude and timing of the maximum paleotemperature so as to define the range of values which give predictions consistent with the measured data. A "multi-compositional" kinetic model is employed, which makes full quantitative allowance for the effect of chlorine content on annealing rates of fission tracks in apatite. This model is calibrated using a combination of laboratory and geological data from a variety of sedimentary basins around the world.

Thermal History Reconstruction (THR™):

THR involves using AFTA^® and VR to identify, characterise and quantify the major episodes of heating and cooling which have affected a well section. Specifically, AFTA is used to determine the timing and magnitude of maximum paleotemperatures in individual samples. VR data are also used to provide independent estimates of maximum paleotemperatures, the timing of which is interpreted on the basis of the information provided by AFTA. The variation of paleotemperature with depth is then used to constrain paleogeothermal gradients, and to characterise the mechanisms of heating and cooling.

Linear paleotemperature profiles with paleogeothermal gradients close to the present-day geothermal gradient provide strong evidence that heating was caused by greater depth of burial with no significant increase in basal heat flow, implying in turn that cooling was due to uplift and erosion. Paleogeothermal gradients significantly higher than the present-day geothermal gradient suggest that heating was due, at least in part, to increased basal heat flow, while a component of deeper burial may also be important. Paleogeothermal gradients significantly lower than the present-day geothermal gradient suggest that a simple conductive model is inappropriate, and more complex mechanisms must be sought for the observed heating. Highly non-linear profiles can sometimes be suggestive of lateral input of heat at a relatively shallow level in the well, perhaps due to hot fluids, or igneous intrusions.

Where appropriate, extrapolation of paleogeothermal gradients to assumed paleo-surface temperatures allows estimation of amounts of section removed by uplift and erosion.

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