ABSTRACT

The TZ-No1 gas field in the Tarim Basin is the largest condensate field in China; however, it has not developed efficiently because of low and unpredictable production during the past 20 yr. Cores, logging interpretation, seismic descriptions, fluid properties, and production data indicate that this Ordovician carbonate field is different from conventional stratigraphic oil and gas fields as follows: (1) the hydrocarbon-bearing area covers a large region of 2000 km2; (2) matrix reservoirs have low porosity (<6%) and low permeability (<1 md) and small throat radii (<1 μm), but superimposed fracture caves with high porosity and permeability are present, resulting in strong lateral poroperm heterogeneity; (3) significant variations in fluid properties and phases as well as the absence of a uniform oil–water contact; (4) except for some “sweet spots” in large fracture caves, economic production requires acid fracturing and horizontal drilling, yet there are many poorly performing wells with complex output of oil, gas, and water; and (5) more than 70% of the production come from fracture-cave sweet spots but with a high decline of production rates (>20% per year). An examination of the carbonate reservoir and hydrocarbon accumulation history suggests that this large-scale stratigraphic accumulation formed during hydrocarbon emplacement in the early Paleozoic, with the reservoirs gradually evolving into tight reservoirs as a result of intense diagenesis. Variable amounts of Neogene gas charged the tight carbonate reservoirs and formed unconventional accumulations. The TZ-No1 gas field is characterized by a strongly heterogeneous tight matrix reservoir with a superimposed fracture-cave reservoir and a complicated unconventional fluid distribution, which provides insights into the exploitation challenges of unconventional carbonate resources.

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