EXPLORATION OF SAND RICH TRIASSIC LACUSTRINE DELTAS IN NORTH SICHUAN BASIN, PEOPLE'S REPUBLIC OF CHINA

David J. Kronman
Burlington Resources, Houston, Texas, USA

Burlington Resources and PetroChina are investigating the exploration and development potential of thick lacustrine high-stand fluvial dominated deltas deposited in the north Sichuan basin during the late Triassic. These sand-rich deposits have wide lateral continuity but detailed examination shows a very complex stratigraphy with substantial diagenetic overprint. Initial prospectivity analysis is encouraging.

Approximately 30 exploration wells have been drilled into the objective “XiangXi” lacustrine depositional section within the original 7,500 square kilometer Chuanzhong block contract area. Two prominent delta sequences have been identified in the XiangXi; both are underlain and overlain by thick (100+ meter) organic-rich low-stand lacustrine shales. Over 1,500 meters of whole core has been acquired in these sandstones, from which 8,000+ core plugs were taken for porosity-permeability and petrographic analysis. 

Studies have primarily focused on the 90-130m thick XiangXi 4 sandstone section that shows uniform lateral gross-section continuity over a very large area (for at least 130 kilometers parallel to the paleo shoreline). This sandstone interval is dominated by stacked braided channels and over bank deposits with surprisingly little shale. Core and wellbore images show the typical section contains more than 50 stacked high-energy channels ranging from 0.2 to 2.0 meters thick (most are erosionally truncated). Grain size range is fairly small and small-scale cross-bedding common. Shale and coal rip-up clasts are also common at the base of channels, but distinct shale and coal beds are rare and discontinuous between wells. 

Deep burial and diagenetic alteration have resulted in an average effective reservoir porosity of 6-8% and average effective permeability near 0.1mD. Thermal maturity analysis indicates maximum burial was >6000 meters. Regional inversion and subsequent erosion now place the XiangXi sandstones near 3,000 meters. Strong geopressures persist throughout the XiangXi section (0.8 to 0.9 psi/ft gradient). 

Detailed petrographic analysis shows the XiangXi 4 sandstone section is mature with 85-90% framework grains and 8-13% authigenics (cements and replacement grains). Using Folk’s (1968) classification, the sandstone is boarder-line Subarkose-Lithic Arkose with 70-90% Quartz, 5-40% rock fragments, and 5-25% Feldspars. Mineralogical composition is remarkably consistent throughout the sampled area. Authigenic clays are the dominant cement type. 

Pore space is dominantly microporosity (approximately 70%), followed by primary porosity (20%) and secondary porosity (<10%). Porosity development and preservation appears to improve slightly distal (north and west) to the delta plain. Extensive petrophysical analysis, utilizing core and wireline log data, indicates the XiangXi 4 reservoir has a large capillary transitional gas column of several hundred meters.

Even though individual channel facies are not correlatable between wells (lack of flooding plain surfaces), two prominent non-stratigraphic higher-porosity intervals in the XiangXi 4 are observed in practically every well across the region. No mineralogical or stratigraphic differences are seen between the higher and lower porosity intervals. Further work is planned to evaluate these intervals.

Gas has been proven down to spill in several large structural closures but small-scale stratigraphic complexity and diagenetic alteration appear to have significantly impaired permeability and gas flow. Wellbore imagery logs and production testing indicate natural fractures are present, but most appear to be sealed. Unstimulated gas well tests have typically been sub-economic, but new hydraulic fracturing stimulation practices have shown promising results. 

The project partnership is planning to drill additional wells in one of the prominent gas-bearing structures during 2004 that will be used, in part, to investigate small-scale reservoir stratigraphy and permeability trends. 

A review of the geologic analysis findings, and technical challenges, will be presented as a poster session.

Significant contributors include: Mike Lentini (currently with Shell Oil Company, Houston, Texas, USA) and Gabe Werda (Global PetroPhysics, Houston, Texas, USA).