The Kingia sandstone of the northern Perth Basin is a prolific gas reservoir that has been the chief target for Exploration in the Basin over the last several years. Multiple fields have been discovered by various operators with flow rates as high as 112 MMcfd derived from reservoir as deep as -4,625mSS. In addition to the existing discoveries, numerous undrilled closures indicate the potential for a multi-Tcf resource.

Of Sakmarian to Artinskian age, the Kingia Sandstone has been penetrated in over 90 Perth Basin wells to date. The Kingia Sandstone was deposited on the margins of an oceanic embayment within a shallow cratonic sag on the Western edge of the Australian continental mass. Well log profiles, image logs and core suggest these sandstones were deposited in a series of fluvial, estuarine and deltaic environments in high order sequences nested in a lower order regression. They form a regionally ubiquitous sand unit ranging from 14-87m thickness deposited in several depocentres along the northern parts of the Beharra Terrace and Dandaragan Trough of the northern Perth Basin. Primary porosity is preserved to great depth by the presence of iron-rich authigenic clays coating quartz grains and occluding authigenic quartz cementation. This porosity preserving mechanism is also supplemented locally by early emplacement of hydrocarbons.

However, the play is not without its problems. All operators have encountered setbacks in their ongoing Exploration and Appraisal drilling, often leading to downward resource estimations. These setbacks may be boiled down to five key elements:

• Presence of perched water aquifers

• Presence of disconnected, erratically distributed pressure cells

• Variations in pore water salinity affecting Petrophysical interpretation.

• Local reservoir heterogeneity

• Regional extent of the play

Presence of perched water aquifers. Whilst the vast majority of deep (Top Kingia depth greater than 3100mSS) wells testing the Kingia sandstones have tested gas-charged reservoir, about one in seven wells have encountered water-wet reservoir, often within the boundaries of gas fields, and in structural configurations suggestive of perched water.

Presence of disconnected, erratically distributed pressure cells. Data to date indicates that the various gas and water legs tested in the Kingia sandstones do not share common pressure gradients. Some small areas peripheral to the uplifted Basin margins (e.g. Lockyer Deep) demonstrate marked overpressures. Others fall on a hydrodynamic gradient. In other areas in the deep parts of the Dandaragen Trough, noticeably different gas pressure gradients and aquifer gradients are observed in adjacent fields/gas blocks. This inconsistency prevents accurate projection of gas legs and gas water contacts in individual fields and fault blocks and introduces key uncertainty in early resource estimates.

Variations in pore water salinity affecting Petrophysical interpretation. Produced water salinity in different wells varies greatly, ranging from over 30,000 ppm to < 2,000 ppm. Selection of the correct water salinity will greatly impact calculated water saturation.

Local reservoir heterogeneity. Several wells have encountered Kingia sandstones that are either thinner than expected, or which contain lower quality reservoir than offset wells. These results suggests that local factors such as lateral and vertical facies variability, variable sediment supply, variable accommodation space evolution, fault associated diagenesis and other factors can introduce a level of heterogeneity upon what was once considered a relatively homogenous, sheet-like reservoir.

Regional extent of the play. The play is bounded to the North by the presence of emergent basement areas (Northampton Block and Pilbara Block). Recent well penetrations to the South of existing discoveries indicate a thinning of overall sand thickness and deterioration of reservoir quality into the deeper parts of the Dandaragan Trough. The southernmost penetration of the play in the Dandaragan Trough is reported to have zero pay. Lack of any well control to the South and East precludes definitive delineation of the depositional limits of the Kingia sandstones, but the overall southward tapering isopach, changes in log profiles and reduction in net pays suggests that a distal pinch-out of the sands into a marine basin, coupled with reduction in proportion of clay coats on sand grains, will probably result in a reservoir zero edge.