BISMARCK, N.D. (AP) — North Dakota's general fund surplus continues to grow at an unexpected pace.

Officials estimate the surplus will be at $457 million when the two-year budget cycle ends on June 30, 2015, according to state Budget Director Pam Sharp. That is a substantial increase from the $87 million projection when the Legislature adjourned last May, The Bismarck Tribune reported.

The current biennium began last July 1. So far, the state has collected just under $2 billion in general fund revenues — more than 6 percent ahead of projections in May 2013.

North Dakota's budget is being fueled by oil wealth. The state has gone from being the country's ninth-biggest producer of crude in 2006 to second, behind only Texas, as oil companies have cracked the Bakken shale formation in the western part of the state. The Legislature last year crafted a $14 billion two-year budget — about $10 billion more than a decade earlier. The amount that came from the tax-supported general fund was $6.8 billion.

State agency budget guidelines for the 2015-17 budget cycle will be released in late April or early May, according to Sharp. By law, agencies have until July 15 to submit their budget requests, though Sharp said extensions can be considered. Between mid-July and October, agencies meet with state budget officials and the governor to go over their budget requests.

The final revenue forecast for the 2015-17 budget cycle will come in November, with the governor's budget address to follow in December during the organizational session for the 2015 Legislature.