HALBUR, Iowa (AP) _ Amoco Production Co. is pushing a exploratory well further below an Iowa farm field than had been planned, leading one geologist to speculate the company wants to explore all the likely oil-bearing rock formations in a major rift across the Midwest.

The giant oil company initially said it would drill the oil and natural gas well 15,000 feet deep, but extended that to 16,500 feet in August.

On Wednesday, the bit was 17,395 feet below the Ed and Marie Eischeid farm near here, and an Amoco spokesman said he did not know how much deeper or how much longer the company would drill.

Amoco is keeping mum on its findings because the test hole is only the second to explore 1 billion-year-old sedimentary rocks in the Mid-Continent Rift, a tear in the Earth's crust from Lake Superior to Kansas. Amoco spokesman Tom Kornegay Jr. said the company's decision to continue drilling does not signal anything.

''The revision of drilling plans and objectives for wildcat wells is commonplace in sparsely explored areas,'' he said. ''For the most part, that's due to the geological uncertainty that exists before drilling.''

Until the Amoco project, no one had drilled deeper than 5,305 feet in Iowa.

Robert McKay, a research geologist with the Iowa Geological Survey in Iowa City, said he believes the company is continuing to drill because it wants to explore the entire range of sedimentary rocks under Iowa and reach the volcanic igneous rocks commonly called the ''basement'' in the oil industry.

Oil usually is found in sedimentary rocks and rarely in igneous rocks, McKay said.

Drilling through the sedimentary rock layers would provide valuable geological information. ''They are interested in reaching the basement because they want to know exactly how deep the sedimentary rock column is,'' McKay said.

The company had said drilling to 15,000 feet would cost $4 million and up to another $1 million to reach 16,500 feet. Kornegay said he could not say how much drilling beyond 16,500 feet is costing, but costs jump dramatically as the hole deepens.